Players choose the creature they going to play but here are some guidelines:
Creature Type
You will start with 4 HD. If you choose a race that has less that 4 HD, you take the difference as class levels.
If you opt for a 1 HD race you can either take the hp for the racial HD or the class HD
For each class level you have, identify one place you have traveled to
Leveling will be story based instead of experienced base so the level differential will not be disadvantage.
You may not select a race that has unrestricted flight (such as a mobat or flumph)
You may not select a race that shape-shifts into people (such as doppelgangers or were-creatures) at will
You may not play a race with animal intelligence or lower (such as an ankheg or ooze). You could, however, play an creature whose attribute bonus gets it up to a 4.
You may not play a race that spawns others of its type (such as wights, chaos beasts,etc...)
Stats
Instead of rolling for stats, you will base your attributes on the attributes of the creature you selected. However, PC are exceptional so you will increase three stats by +4, +2, and +2. If you play an undead or other race that does not have a stat, you get a +4, and a +2.
Your first HD will be maximized for HP. All other HD will be half HD, rounded up. Unless you are a 1HD creature, your first maximized HD will be based on your race.
Feats- You do not start with an initial feat. You do start with any feats, if any, that are listed in the creatures' statistics
Traits- You do not start with any traits. You can earn them through the Extra Traits feat if you choose to take that feat at a later time.
Attributes and Abilities- You will have listed bonuses like Natural Armor, Scent, Climb, Swim, Spellcasting, Supernatural and Extraordinary abilities, etc... Of course I may cancel any I think are problematic..cause you know, Lamashtu is like that.
Skills
You will have the innate skill bonuses listed for your creature type. Your character is proficient in what ever skills are listed with a positive bonus (which is assumed to be factored in already). In many cases the skills will have to be recalculated and may be lower than in the SRD.
You have learned Common, if you don't already know it.
Alignment- No one may play a good aligned character
Death- There is the likely possibility that PC are not as supportive as a traditional campaign and that might result in PC deaths. If your character dies, your next character will have an additional +1 to a stat or a +1 to a particular saving throw. This bonus will be cumulative.
Depravity- What is your character's psychological drive to adventure? It could range from the ordinary, such as accumulate power or money, to the extreme, such as hatred for the living or spread fear. This is your excuse to act against everyone's (even your own) interests.