Goodness! Too many

Allen:
Da King, Orange guy, big guy, bub, Dog-Cat (since he thinks he's a dog)

Pouncer:
Pounce (gee, that one took some thought!) Bud, Hissy

Harry:
Bud (poor Pouncer and HGarry share a nickname) Hungry, Dude

Abby:
Princess, The Girl, My Girl, Abs

Flutter:
Flutter Butter, Squeakheart, Squeaky, Squeaks (notice a trend here?) Little One