Why are characters named Momonga named after flying squirrels?
In One Piece, the animal name is the most typical source of the character's name. You can read how the author decided the name 'Monkey' in the manga #20 Chapter 178's SBS.
The origin novel of Overlord is written on a free novel BBS Arcadia. You can read how the author decided the name 'Momonga' on the following 1st chapter (Japanese) (I'll translate just the conclusion):

※　名前でモモンガは無いですよね。
でも、普通にネットゲームをしている社会人の方なら、基本的にちょっとおちゃらけ入った名前の方が多いような気がするんですよね。私だけですかね？
口調とか、名前とか変更するときがそのうち来る予定です。
※ Momonga is a strange name.
But many working online gamers choose the strange name, I think. Just me?
I'm planning to change his tone and name.