My first name is Ken, and throughout childhood I was known as Kenny. That really grated on me in high school so I asked people to stop. Currently my wife and a few old childhood friends call me Kenny, but no one else does.
As a counselor at Boy Scout camp, I was nicknamed Scooter by one of the other counselors. No reason; he just felt I needed a nickname and that was what he came up with. I was known as Scooter among any number of Scouts for a few years. (His first idea was Hooter; I think I dodged a bullet there.)
When Mrs. Silvercrest and I were dating she compared me to a turtle because (as I say to others) I'm stubborn, one-tracked, and slow. I don't actually use that as a nickname, but I've adopted a turtle motif in a lot of things and I've made a collection of small turtle statuettes. Occasionally I refer to myself as a turtle and once in a while someone calls me one.
After Mrs. Silvercrest developed some health problems, we agreed that she would be "snail" because it was the only "cute animal" that's slower than a turtle.