It's a native field rat, bigger than a mouse, not as big as a town rat, but nearly as destructive. We're surrounded by pasture land and they show up in the yard from time to time. Did you notice the couple of crickets roosting on its back? The wind blew the flip-flop into the pool so the rat may have spent time swimming before providence provided a raft. It was a chilly morning, too.
I netted and flung it over the pool fence. It crash-landed in a clump of grass and vanished after resting a few minutes.
I grew up calling them thongs, too. Don't do that anymore.