There are several in Stygian Abyss. I recall that we even had a developer meeting specifically to design them. Of course, I can't give details... otherwise they wouldn't be Easter eggs.
There are mobs that have a random chance to say funny things. Items that when used have a random chance to polymorph you. Stuff like that. None of them are there to change the balance of the game, that wouldn't be an Easter egg. They are just rare events that are meant to entertain.