Hi WJ... sorry, I meant to reply to this yesterday but forgot.
It's hard to see but I vote for prolapse as well, I've seen it a few times but have never been able to cure it. Sorry you lost him
I've also had isolated cases of dropsy in a crowded tank (Endlers), where no other fish were affected. Tank was well established and water tested ok, apparently it can happen out of the blue sometimes. Sorry about your guppy also