Kero-chan is appropriately named "Kerberos" after the beast that guards the entrance to the Underworld in Greek mythology (more commonly known as Cerberus).
Although "Kerberos" is closer to Kero-chan's Japanese name "Keruberosu" (
), CLAMP's official English name for him is "Cerberus" (you can see his name written in Japanese at the top and in English at the bottom in his official profile, form vol. 2 of the manga). I find this pretty strange, since "Keruberosu" is nothing more than "Kerberos" written in Katakana. But I guess "Cerberus" is easier to recognize...
Anyway, Kero-chan received his nickname when Tomoyo met him and mentioned that a big name like "Keruberosu" didn't fit his appearance. So she and Sakura decided to call him "Kero-chan".
To explain it poorly, "chan" is an honorific attached to young children's names or to the names of cute or small things (although it's not restricted to just that). It's a sort of diminutive, and people often attach it to the names of their stuffed animals or pets. Needless to say, Kero-chan (so fond of making it known that he is "The Beast of the Seal, Kerberos!") was not particularly pleased with this. Especially since "Kero" is a typical name for pet frogs ("kero-kero" is the Japanese equivalent of "ribbit-ribbit") Ever heard of "Keroppi?" ^_^;
Nevertheless, Kero-chan never really complains about his nickname, and it fits him very well (I'm sure he's happy as long as at least Syaoran refers to him as "Keruberosu" sometimes).
