DM's GUIDELINES FOR PLAYER-CHARACTER MONSTERS
Fit the monster race, as best as possible, into normal format for human, demihuman, and humanoid PCs (normal ability scores, classes, kits, proficiencies, etc.).
Examine possible means of communication, movement, manipulation, spellcasting, and skills acquisition; think about special advantages and disadvantages in play.
Allow only classes and kits that seem logical and sensible within the confines of your own campaign and common sense.
Allow for increase in size and ability scores for animals and monsters if they grow from juvenile size to adulthood. Consider setting a creature's maximum level limit at twice its normal hit dice, for most sorts of animal PCs.
Consider allowing weapon proficiencies for natural weapons and attack routines (claws, bite, head butt, hug, dive, spit, throw item, etc.), with bonuses to hit and damage being added to normal combat results. The natural weapon proficiency from The Complete Book of Humanoids should be considered.
Clarify all vague powers and areas of ability, getting the exact effects of innate magical powers, proficiencies, etc. Carefully consider unbalancing effects of certain special powers, and look for ways to restrict those special powers without removing them.
Look at similar creatures in MONSTROUS MANUAL tome or other game or literary sources for possible powers, abilities, and social notes. If a real-world animal is being played, look up authentic material on its appearance and behaviors, modifying game material if desired to add more realism and special traits .
Establish and stick to limits on playability for your particular campaign (no undead, no Outer Planes critters, no giant space hamsters, no beings over 16 hit dice or 16th level, whatever).