I know the spells themselves don't require "familiarity" with the creature you are turning into, but my GM likes a certain level of common sense at his table. If you have never heard of Creature X, why would you be able to transform into one, and gain abilities you didn't know it had in the first place?
What do you think would be a suitable Knowledge DC to "have heard of" Creature X to turn into one? Would you use the Knowledge Skill "Identify a monster's abilities and weaknesses" DC of 10+CR? Or something a little easier just to have heard of it (like 5+CR). If it has multiple abilities (like Grab, Energy Resistance, Vulnerability, Freeze, Pounce, etc.) would you get them all with a successful check or do you think the "for every 5 in which you exceed the DC you learn another ability/weakness" would be reasonable/unreasonable?
This would make it advantageous to "research" creatures you are interested in ahead of time to make sure they are available when you get the spell needed (Monstrous Physique, or Beast Shape, etc.)
Well, I would just use the DC to identify a creature. If you can successfully identify a creature, you are familiar with it.