Well, I do it as AB said, and I also did it just for fun sometimes. I had a TB registered as Rippling Jiggs. YUK!
I have not had one registered since, but my clyde cross came with the name "Second Chance", barn name "chance". I changed it to Frank, and used the "second Chance, but when my daughter did 4-h, she called him "bob's big boy (DH is Bob). Didn't matter, it was just fun.
The newer one, QH/TWHcross I have registered with the NRHA, so I needed a "show name". He is "Gotchur Six" to them. He will accrue points (if we should ever be so lucky!) under that name.
I have seen some H/J people name them fun names even if they are, for example registered QH-totally seperate point system.
Signature undergoing edits. Please standby.......