LifeOnWry 
Some created their handles as enthusiasts or hobbyists. Tuckerfan springs immediately to mind. Several chose names related to beer and winemaking.
A broad category of “literature based” would also be appropriate. The Hitchhiker’s Guide and the Principia Discordia (from which I took my handle) are good examples with a few variations of Ford Prefect, including a combo name in Fnord Prefect.
Hmm… How about just going with “Animal Based”? (The word, anthropomorphism, has been overtaken by furries.)
Though the rat segment is doing pretty good, we have:
Sage Rat
Duke of Rat
GiantRat
And me! Tengu - Wikipedia
(It’s also a brand of clam jerky, but…let’s not go there.)
I’m an animal!
So are wolf_meister and Unregistered bull.
Historical figures
“featherlou” is well-placed in the “Lou” category, but as for the “feather” part, is there a category for inanimate objects? I could also go in the category “Named after pets.”
ZipperJJ = Squirrel Nut Zippers = Musical (they’re a band)
And I coulda swore Argent Towers was musical too - Rod Argent. “Color” me enlightened now 
Maybe names from science fiction books should be a subcategory of “literature based.” Lazarus Long, Podkayne and teela brown are good examples.
I could be flora
Domain: Internetari
Kingdom: Dopera
Phylum: Plantae
Class: Sporofolia
Order: Silva
Family: Duonomenae
Genus: Capitala
Species: Fern Forest
I was wondering when we could do this again. It’s been 5 years since the last play through ended.
Do we have a category for “kitchen flatware”? If so, I’m there!
You could also be ‘sleeping positions’.
Do I fit here?
But if I were to pick my own category, I’d have to go with Religion (Or Lack Thereof). 
Yes, true enough. I guess I didn’t think of that because my username did indeed come from the flatware.
Woo-hoo! Two categories for me!
How about a workplace name? I really do work at a zoo!
Three! You can even make it into musical! (I can’t play the spoons, myself, but I know people what can.)
Apocope for location counts? I took off the -rra at the end.
I like it. I’m both an engineer and a nerd. Also occasionally redundant.
You called?
Oh, wait…