on bulbapedia it says: Previously, due to the censor that prevents Pokémon with offensive nicknames being traded on the GTS, an English language Cofagrigus could not be traded without a nickname. But why.
Because there was a bad word in his name.
Cofagrigus
Since all names had to be clean, and this one wasn't, it was deemed to be nicknamed forever. :o
Hope I helped. :) Source: Experience