I named my naked neck Fifi because she looks like she has a poodle hair cut.
Other than that, its by personality and/or looks.
Chirpy
Toadie
Lola & Sam
Puny & Fluffy
Big Red
Blondie
Fiona
Scrappy Do
Grover
Princess
Lucy
Wobbles
Ethel
Babs
Thelma
Etc.