Fire Golems in the Nether that attack you, ones that don't attack you but attack monsters can be made as well. Are like Snow Golems but shoot fireballs. Could be made with 2 netherrack and a pumpkin, or 1 soul sand, 1 netherrack, and a pumpkin.
Bees in sunflower plains, or are able to spawn around a group of sunflowers anywhere.
A rare flying horse that can only be spawned by breeding or using NBT. (Pegasus)