One of the best parts of my job is finding things like this incidentally in a body!!! I have only come across one horseshoe kidney in my career, they are pretty rare. When I was doing the autopsy I was trying to pop the right kidney out of the capsule to weigh it, but it wouldn't pop out! When I looked deeper in the body to investigate the problem I saw that the right kidney was attached to the left kidney!!! I was so excited! A horseshoe kidney!! I love embryologic anatomic "screw ups", and by that I mean when something goes wrong with a human as we are developing as a fetus.
Rarely, in fetal development, the right and left kidney can fuse causing a horseshoe shaped kidney. People can have one their whole life and not even know it. They are set up in the body in such a way that they can cause problems like fluid accumulation in the kidney and kidney stones. The biggest problem with a horseshoe kidney is- you only got one! My grandmom always used to tell me she had one kidney. I always blew her off and was like ahhhh whatever. When she got sick- I read her CT report and in the report it said "horseshoe kidney"