This is a shot of the rainbow colored pigeon that Redditor chesterpoops's (everyone does, it's no big deal) girlfriend took while in London. Some people speculate it flew through one of those color run races where they sprinkle you with rainbow fairy dust to make you run faster or more visible to traffic or whatever. Other people speculate its a Spanish racing pigeon, which are handpainted by their owners before a race or being released to find a mate. Me? I'm a huge fan of the simplest answer being the right one. "Which is?" That's a duck.
Thanks to Andrea, who agrees it probably escaped from an alternate reality where there's a rainbow version of every animal on earth.