That is one mutant-looking Rainbow! It looks like some of the "Croakers" I've seen pictures of, which originated in farming operations. They are fed enormous amounts of food, and have that same body shape. The World Record Rainbow was a Croaker.
A lot of anglers disagree with allowing fish that originated in farming operations to be considered as World Records. Here is an article about the current World Record Rainbow. The guy who caught it, and his Brother, specialize in catching giant Croakers. You can do a google search, and read a LOT about them.
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