(Request Punchline Help) In the Serengeti, a male rhinoceros goes to take a drink from a river when a hippo pops his head out and says, “Hey Rhino, you gotta help me, man!” “What’s wrong?” says the rhino. “It’s my mate. She wants to have another calf and we’ve been doing it all day and night...

"...I’m a hippo! I don’t have that kind of energy! I need a day off, dude. Could you crawl into the water and pretend to be me just for a day?”
“You think your wife won’t notice?”
“Come on, we’re both big, grey, leathery things. She’s so out of her mind with lust, you should be fine.”
“Well, to be honest, I do think hippos are kind of sexy and curvy and I am kind of a freaky rhino. Sure, let’s do it.”
They switch places and the hippo trots over to a shady spot and has a nice relaxing day off.

The next day he returns and the rhino practically leaps out of the water to escape. Half his nose horn is broken off.
“I can’t believe you talked me into this!” shouts the Rhino, running away. “Goddamn!”
The hippo shrugs and plops down into the river and finds his mate.
“Hey stud,” she says to him, “Ready for more?”
“I can’t believe it,” says the hippo. “That was a rhino you were with all of yesterday and you didn’t even notice!”
“Huh.” says his mate. “That does explain why you were much hornier than usual.”

(note: I feel like there’s probably a stronger punchline here. If you can think of one feel free to drop it. Something along the lines of: That was a rhino? I thought you brought an anaconda for a threesome.)