So many folks out there in the blogosphere spend time deconstructing and expounding upon the 1e DMG's Appendix N, but we don't give the same weight to the the "Inspirational Source Material" found on page B62 of the 1981 Basic Rules. It is broken out into five sections: 1) "Fiction: Young Adult Fantasy," which includes Lloyd Alexander, L. Frank Baum, Edgar Rice Burroughs, and C.S. Lewis, 2) "Non-Fiction Young Adult," 3) "Fiction: Adult Fantasy," which includes most of the material from "Appendix N," including de Camp, Dunsany, Lovecraft, Tolkien, and Vance, 4) "Short Story Collections" and "Non-Fiction." Even through the likes of Baum and Lewis have been noted, their influence is all-but-absent from the game we know (even the classic D&D version of it). So here's where I'm going with this...
Because I've been in such a "character class" mode lately, I was giving some though to intelligent creatures as player characters. Ultimately, the goal here is a systematic approach that lets GMs "version out" intelligent animals based on their campaign. Also, given the BX version of the "Appendix N" noted above (and the lack of similar references from the 1e DMG), I'm going to keep these within the BX/LL concept of race-as-class.
Almost any type of normal animal should be available as a PC class. The types of animals available to any given campaign should, however, be limited by the campaign (obviously). There's no reason that mice, or birds, or leopards, or even crocodiles shouldn't be available as PCs.Click here for the followup post to this one.
Classed intelligent animals will come in two basic forms: Option A) a standard version of an animal (i.e., no physical differences between it and its standard counterpart), but with the ability of human speech, and Option B) a slightly anthropomorphized version of the animal, also with the ability of human speech. The important word in that second option is "slightly." Anything more, and dogs might as well be gnolls, and lizards might as well be lizard men. Think puss-in-boots, or Reepicheep from Prince Caspian. The first type will not be capable of wielding any sort of weapon or shield, but armor options should be based on the type of animal (e.g., there is no reason a horse shouldn't be able to wear plate armor, if someone is there to put it on for him, of course). The second type will generally be capable of using weapons, shields, and other objects/tools (within reason), with armor type being limited by the size/strength of the creature (e.g., a mouse might be too encumbered by any type of armor, while a possum might still be capable of fighting in partial plate).
XP advancement for intelligent animals will be limited to a certain level. I'm seeing "Option A" types as having a much lower level limit than the "Option B" types. Maybe somewhere from 7th to 9th level vs. somewhere from 10th to 12th level, respectively.
"Option A" Special Abilities. I see them having the ability to speak with common species of their type (e.g., brown bears may speak with grizzlies, black bears, etc., while most great cats may converse with one another). They may not, however, speak any other languages other than their animal language and common (though I'm considered re-examining this). At a certain level (maybe as early as 3rd), some animal types may attempt to engage an assistant (most likely human) as a paid hireling/henchmen (I don't know about you guys, but sometimes I wonder who the master is... me or my dogs). These types of intelligent animals will also have the ability to summon like types from the area to assist them; this will be an increasing percentage chance of response with an increasing range per level (think Thieves' Abilities here). Some animal types will also have a special magic-like ability with an increasing chance of success per level (again, a la Thieves' Abilities) or as a magic-like ability that improves (range/duration) with each level (e.g., a leopard might have a paralyzing gaze attack vs. an individual, a black panther may have the ability to hide in shadows, a bear might have a fear attack, etc.). At high levels (for their type, anyway), they may establish a "kingdom"; this is really nothing more than an area of land over which the animal rules, but he or she will attract "subjects" who are loyal enough to fight for them should the need arise.
"Option B" Special Abilities. Unlike their "Option A" counterparts, I see "Option B" intelligent animals having to learn their species' language, just like learning any other language. Also unlike their counterparts, I see the "Option B" animals being able to learn other languages (humanoid, demi-human, or even monster languages). They will not have the summoning ability of their "Option A" counterparts, nor will they have the same magic-lie abilities. However, I do see certain types of animals being able to learn certain class abilities (e.g., maybe at a certain level, some have access to charm spells, or some may have Thieves' abilities which increase with each level). At high levels, they will be able to establish a traditional stronghold and attract followers of their same "class" (e.g., an "Option B" mouse will attract low level types of the same mouse class).