My brother calls my cousin "Water Head" because his head is shaped a little bit like a water bottle (the gallon size)
For some reason, my aunt is called Trudy by her sisters and her father and Tootie by her nieces and nephews (we thought the older people were saying Tootie when they called her Trudy). Anyway, her name is Caroline. No one seems to know how that got started.
And of course the classics: Rae-Rae (girl cousin), Janie (mommy MJ), Ty (my brother Tyrone), and Pop-Pop (y'all know already)