Here's the list of creatures corpses you can carry in your backpack:
Bat, Badger, Black Sheep, Sheep, Cat, Deer, Demon Skeleton, Crypt Shambler, Dog, Chicken, Frog (All types), Rabbit, Rat, Fire Devil, Minotaur, Minotaur Mage, Minotaur Guard, Snake, Elf Scout, Elf, Elf Arcanist, Poison Spider, Spider, Troll, Troll Champion, Wolf, Island Troll, Crab, Scorpion, Cobra, Mummy, Pirate Skeleton, Skeleton, Ghost, Pirate Ghost, Ghoul, Swamp Troll, Frost Troll, Seagull, Cave Rat, Butterfly (any color), Bug, Parrot, Wasp, Goblin, Goblin Assassin, Goblin Scavenger, Orc Marauder, Orc Shaman, Orc Leader, Orc Warrior, Orc Spearman, Orc Berserker, Orc, Orc Warlord, Larva, Pig, Dwarf, Dwarf Solider, Dwarf Guard, Gnarlhound, Dwarf Geomancer, Sandcrawler, Winter Wolf, Vampire, Scarab, Skunk......
I'm going to test some others these are just the ones I've tested so far. As a general rule corpses which are larger than 1 square or 32 x 32 pixels can not be carried. An exception to this are Troll corpses, which are 40 x 32 pixels in size. Please note fresh corpses can't have anything inside them in order for you to take them. You also can't move corpses into your backpack until 10 seconds.