In this post, I will default to the style of capitalizing in-game terms with defined meanings. This is most relevant example is the difference between animals (the normal scientific real-world definition of "members of the animal kingdom") and Animals (the D&D 3.5e definition of "members of the Animal Type").
In Dungeons and Dragons, one rarely encounters actual living animals, except when a local cat elects to leap up on the table to play with the dice and miniatures. However, the game's treatment of fictional animals reflects our thoughts about real animals, and is thus worth contemplating.
--- Animals and Types
In D&D 3.5, all nouns are either Creatures, Objects, or Conditions. (There's some debate as to which category some things fall into and whether a thing can be both -- for example, it's not entirely clear whether a corpse is an Object or simply a Creature with the Dead condition. While it is clear that Intelligent Items are Creatures, it's not clear whether or not they are also Objects.) Note that the Creature/Object dichotomy is not the same as the Living/Nonliving dichotomy -- there are nonliving Creatures (such as zombies and golems), and there are living Objects (such as trees and bushes). A Creature is defined as anything with a Wisdom and Charisma score. A Condition is something that affects a Creature or Object, such as Paralysis or Disease.
Of the fifteen Types in D&D (Aberration, Animal, Construct, Dragon, Elemental, Fey, Giant, Humanoid, Magical Beast, Monstrous Humanoid, Ooze, Outsider, Plant, Undead, and Vermin), all real-world animals, past and present, are distributed between three: Animal, Humanoid, and Vermin.
The only real-world animals with the Humanoid Type are humans and neanderthals (Neanderthals are treated as somewhat of an afterthought in the D&D canon, mentioned only in one of many supplements, so I will mostly treat them as an afterthought as well). Other Humanoids include elves, orcs, dwarves (which are distinct from dwarfs), gnomes, halflings, and so on.
Vermin comprise, more or less, invertebrates. Most Vermin species are fictional, including such varieties as spiders, centipedes, and scorpions ranging from Tiny (the size of cats, just this side of realistic for some examples) to Colossal (the size of a smallish house). Vermin are often Mindless, which means they have no Intelligence score. More on that later.
The Animal type comprises, more or less, all real-world non-human animals, past or present. There are a few fictional species in the Animal type, mostly limited to the Dire Animal category (Dire Animals being creatures that are larger and stronger than their regular counterparts, but which still have the traits and features of Animals). From cats, dogs, and horses to apes, sharks, elephants, squids, and so on, the Animal type is what I will mostly be concentrating on today.
There are some animals that might not be considered Creatures at all. Tapeworms, for example, are not (to the best of my knowledge) given statistics or game effects anywhere in published materials, but if they were, I would lay at least even odds that it would be treated as a Disease (i.e., a Condition) and not a Creature.
The most salient trait of the Animal type is "Intelligence score of 1 or 2 (no Creature with an Intelligence score of 3 or higher can be an Animal)." (I could also run with "Alignment: Always Neutral", but that would make for a much more ethics-heavy post.)
--- Animals and Intelligence
In D&D 3.5, every creature has six Ability Scores: Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma. The most morally salient of these scores is Intelligence (and, to a lesser extent, Wisdom), so this is what I will be concentrating on.
"Intelligence determines how well your character learns and reasons." It also determines how many languages you know (a Creature with an Intelligence of 2 or lower knows no languages; most Creatures with an Intelligence of 12 or higher knows more than one language.)
"Wisdom describes a character’s willpower, common sense, perception, and intuition. While Intelligence represents one’s ability to analyze information, Wisdom represents being in tune with and aware of one’s surroundings. [...] If you want your character to have acute senses, put a high score in Wisdom. Every creature has a Wisdom score."
Humans have ability scores ranging in a bell curve from 3 (abysmal) to 18 (just this side of superhuman), with the mode, median, and and mean being 10. The most common method for generating ability scores for normal humans is to roll 3d6 -- which is to say, roll three 6-sided dice and add the results together. This approximates the given range and average. (Exceptional humans -- such as most player characters -- use other methods. Non-humans use the above method and then add Racial Modifiers to some Abilities and subract Racial Modifiers from others. For example, an elf adds 2 to their Dexterity but subtracts 2 from their Constitution, so an elf would have 5-20 Dexterity but 1-16 Constitution.) (Intelligence, being a range with a bell curve, can be mapped to IQ, but doing so is beyond the scope of this post.)
A Creature can have a Nonability in some score, which is usually indicated by "--" or "Ø". A Creature that cannot move, for example, has a nonability in Dexterity and Strength. A Creature that is not alive (such as a zombie) has no Constitution. A Creature that cannot think and is as an automaton has an Intelligence nonability. (Note that this is distinct from having 0 in an Ability. A Creature never naturally has 0 in an Ability, but it can have its Abilities reduced to 0 through Ability Damage or Ability Drain -- in which case it is paralyzed, unconscious, or dead, depending on the Ability affected.)
Which brings me back to "Intelligence score of 1 or 2 (no Creature with an Intelligence score of 3 or higher can be an Animal)." If an Animal should gain Intelligence of 3 or higher by any means (usually magic), it ceases to be an Animal (the rules are unclear on what it becomes, but many players hold the interpretation that it becomes a Magical Beast). Similarly, Vermin are usually Mindless (they techniacally can have Intelligence scores, and can even have Intelligence in excess of 2, but all published Vermin have a nonability in Intelligence), meaning they are no better than automatons.
This means two things with which I intend to take issue: First, that the most intelligent Animal is considered less intelligent than the least intelligent human; second, that the wide range of animal intellects is confined to the range of --, 1, and 2, whereas the (perhaps relatively narrow) range of human intellects is given the broader numerical range of 3-18.
Dogs, octopuses, corvids, parrots, monkeys and apes, elephants, dolphins, and rats are all widely considered very intelligent, capable of tool use, complex communication, and problem solving -- quite probably more intelligent than many very young or severely handicapped humans. And yet these creatures are all considered to have the absurdly low Intelligence score of 2.
Bees are hardly automatons, being capable of complex "dance" to communicate the precise location of desirable food sources. Other eusocial insects, such as ants, termites, and some wasps, are similarly gifted in organization. Some spiders are capable of weaving complex webs in unlikely places, or of lying in ambush. A defense of the "Vermin are Mindless" school of thought might be to observe that these behaviors are instinctual, preprogrammed into these animals by evolution, and not learned -- which is probably fair, and I don't know that I have a strong position on the subject one way or the other.
The numerical range of --, 1, and 2 is much narrower than the numerical range of 3-18. The game's focus is, naturally, on playable characters, so it makes sense that the range of human experience would be given finer degrees of distinction than the range of animal experience, but I would argue that the difference between the least intelligent human and the most intelligent human is less great than the difference in intellect between, say, a leech and an elephant.
The Wisdom of published animals, on the other hand, ranges from 8 to 17 (with an average of just over 13), so the general consensus at Wizards of the Coast appears to be that the average Animal is wiser than the average Human (though the wisest human is slightly wiser than the wisest Animal). This undoubtedly has something to do with Wisdom's connection to the senses -- many animals certainly have a variety of keener senses than humans do. It likely also reflects the perception that animals are more "in tune" with nature than most humans are (Wisdom is the primary ability score for Druids and Rangers, the two classes designed around being in tune with nature).
--- Animals as Trade Goods and Carrying Capacity
Animals also appear in the section Wealth and Money, under Trade Goods. In the same chart as "one pound of wheat", "one square yard of linen" and "one pound of platinum" are entries such as "one chicken", "one pig", and "one ox". This serves to emphasize the use of animals as objects, rather than subjects.
Similarly, it is possible to buy a mule to carry your loot. With a carrying capacity of up to 690 pounds and a cost of only 8 gold pieces (less than a pound of saffron or a flask of acid), mules are among the best methods to carry stuff around (until you start picking up extradimensional storage space like Bags of Holding and Portable Holes). This reflects their use for this purpose in the real world, but barely acknowledges their status as living beings, much less their status as beings of moral concern.
--- Animal Companions, Special Mounts, and Familiars
Some Classes gain a companion animal as a class feature. Druids and Rangers gain Animal Companions, Paladins gain Special Mounts, and Sorcerers and Wizards gain Familiars. These all begin as regular Animals but gain features as the character with which they are associated levels up.
Animal Companions are drawn from a list of Animals including wolves, owls, badgers, snakes, and so on, and never gain intelligence, though they gain strength in other ways.
Special Mounts begin as warhorses or warponies, but they are treated as Magical Beasts instead of Animals.
Familiars begin as animals such as bats, cats, owls, ravens, and weasels, but they become Magical Beasts when they become Familiars, and they gain, in addition to other abilities, Intelligence. This means that, at very high level, the Wizard's or Sorcerer's Familiar might wind up being among the most intelligent creatures in the party. And yet, despite being the most intelligent creature in the party, the Familiar still tends to be sidelined in favor of the main player characters.
--- Handle Animal and Wild Empathy
There are two main ways of interacting with Animals.
Most characters use the Handle Animal skill, which deals primarily with training animals to do Tricks (such as "Attack", "Heel", or "Track") and "pushing" them to do Tricks they don't know.
Druids and Rangers, on the other hand, can use their Wild Empathy class feature, which allows them to improve the disposition of Animals towards them, for example to make a Hostile Animal Friendly. This functions in the same way as the Diplomacy skill works on creatures with an Intelligence of 3 or higher.