Max: Maximillian, Mac Dog, Mighty Max, Moose...
Lumpy: Actually "Lumpy" was a nickname that stuck! He was originally "Mordecai". As far as now nicknames? The Lump, Bumper and Bumpus Hound.
Willie: Wonka or Sniller
I'm so glad it's not just me Gilbert answers to - BooBoo, poopydoop (I have no idea where that came from lol), Bert, Wilberforse, wilhamina and - the most embarassing one - Wilma. Embarassing because 'good boy Wilma' really confuses people