Great question. I definitely would not assume that the spell gives you any knowledge of animals, no. That’s the road to madness.
If you need an objective way of handling you might try something like this:
- Reference the Wandering Monsters in the Wilderness table, Subtable: Animals
- Select 2 terrain types representative of the character’s background and upbringing. These are the animals the character is intimately familiar with.
- Select 2 terrain types representative of environments quite unfamiliar to the character. (Prehistoric and one other). These are animals the character would have to encounter in-game to be able to transform into as he’s not familiar with them at all (unless they are on the prior list).
- All other animals the character can transform into if he makes a Naturalism proficiency throw 11+ to remember them, or encounters them in game.
- Ignore any references to monsters that don’t count as animals (such as spiders).
EXAMPLE: Shem is a desert shaman. He is intimately familiar with barrens and deserts, so he can transform into Antelope, Camel, Cave Bear, Dire Wolf, Eagle, Gecko Lizard, Giant Hawk, Hawk, Lion, Mountain Lion, Pit Viper, Rattler, Rock Baboon, Tuatara Lizard, Vulture, Wild Dog, and Wild Goat. He is unfamiliar with Prehistoric and Jungle and cannot transform into animals such as Boar, Giant Shrew, Python, Mastodon, Monkey, Spitting Cobra, or Tyrannosaurus.
We’re deep in house-rules land, obviously.