I nickname all my Pokemon. :3 The name usually depends on what I think is suitable for the Pokemon in question.
Prior to Leaf Green, however, I had preset names for certain Pokemon, typically after mythological deities (i.e. such as naming a male Espeon, Apollo or a female Umbreon, Artemis). The reason for this was because I didn't have anyone to trade with at the time, nor did I have a link cable to trade between my games at the time, so I re-used nicknames of Pokemon every time I reset the game to give myself the impression that I was still playing with the same Pokemon as last time.
Eventually, however, as new trading methods were placed into the games (particularly the migration feature), it came to the point where I often kept the Pokemon with the original nickname and refuse to name another after it, because the character for that Pokemon is already defined in my head. It's kind of like having two dogs of the same species with the same name, to make a comparison. It doesn't quite feel right. :/