Happy's name was originally supposed to be Freyr. Hiro felt that it didn't quite feel in tune with the character's personality so it was eventually changed.

Yay Happy my 20th favourite character after Erza, Levy, Lucy, Juvia, Cana, Mira, Evergreen, Wendy, Carle (?), Grey, Natsu, Elfman, Laxsus, Freed, Bikslow, Lyon, Natsu, Romeo and Loke