Animal Companion Hit Dice

[FishBonePendant]

I was building a Spirit Shaman to replace my dead ranger in a campaign (Oh sure, the vegan ranger was a headache for the party, but the vampire bugbear assassin is just peachy though), and was looking into taking Wild Cohort to get an animal companion the same as my spirit guide. After reading through both the feat description and the Druid animal companion class feature I'm confused.

Do all animal companions start at a specific HD or can the character just pick whatever is listed in the creature's advancement? I can't exactly find the rules in the few books I own.

[Bronk]

You start by picking an animal based on what level you are and the animal starts as an average specimen of its type, meaning that its stats are those of the animals stat block with no changes. After that, the animal advances not by its own animal hit dice but through the additions granted by the feat (or class feature) based on your level.

The hit dice are real, and grant feats, but aren't racial hit dice and don't grant size increases (for example).

The rules for a druid's animal companion are down the list a ways here:
http://www.d20srd.org/srd/classes/druid.htm

The rules for the Wild Cohort feat are here:
http://www.wizards.com/default.asp?x=dnd/re/20031118a

[FishBonePendant]

Well that's a shame, there's nothing like rolling a die to see if you can get a larger raccoon for a pet?

And wouldn't the companion gain XP and race levels through encounters?

[Bronk]

Well, you could trade in a smaller base animal for a more powerful base animal at higher levels, although the bonuses they would get based on your level would be fewer.

Edit:
Oh, and no, they don't gain XP on their own, they are treated as one of your class features instead of a regular NPC ally or fellow PC. All of their gains come from your level, and if your character let them go they would go back to being a regular animal.

[FishBonePendant]

Thanks for the help, looks like me and this 3HP Raccoon are gonna be really close for a few levels.

Edit: oh wait, I'm level 7. This is great!

[Bronk]

Good luck!