My mares registered name is horrible. The previous owner didn't like it either and so gave her a short name that was easy. After I bought her and started riding her - her nick "endurance" name is Powerhouse. Because that is what she is!
I have another little mare, an 8 y/o still green though and not sure what she'll end up doing. This mare also was given a not so great name. I wanted a positive name so named her Rosie. So when she is good, I call her "Rosebud" but when she is little stubborn she's called "Snots". LOL, in a loving way of course.
She's actually gonna make a nice horse one of these days.
My husbands horse was renamed Maverick cause he loves that movie. He's a big boned, heavyweight(for heavyweight rider) grey gelding. I rode him today for only the second time ever and decided his name should be "Big Fat White Boy" cause he is...:) lol..my daughter and I did wonder however, how that would go over on his vet card if I ever do a ride on him!! :)