Mice and rats are still a popular choice. Where do you think all those dire rats that get stuck in NPC's basements come from? Not to mention the wererats, ash rats, moonrats, alchemice, ROUSes...
And judging by the evidence, owls were at one time in vogue. It is my hypothesis that Thesselar invented the owlbear after running out of things to do with his poor lab-owls.
Trolls and other regenerating beasties are perfect for anatomical studies - but they only teach about their own anatomy, obviously.
Demiurge out.