Hippopotamus

I made my level 5 players fight an undead Hippo and it OHKO'd an NPC follower. I kinda love how scary this thing is. 

The hippo is so tanky because they share the elephant's ridiculously thick 2-inch hide while ALSO having unusually dense layers of muscle and fat beneath that, creating an impact-absorbing barrier that most firearms can't even punch through. Combine that with their thick bones, and you've got potentially THE tankiest animal on the planet.

In ancient times hippos were so scary, it's where the Bible's Behemoth comes from. "A beast so mighty only God himself could overpower it."

Feels like it deserves some sort of additional effect when it bites stuff, considering it's got a mouth like a hydraulic press... like added bludgeoning damage or an auto-grapple or something.

Quote from Nikcantsnipe >>

Rhinos are not stupid. They cannot easily be scared to back down. That's the opposite of what Rhinos are. They are nearly blind and attack each other. They don't even back down to elephants. Might want to brush up on your animal facts. An elephant is not even close to being twice the size of a hippo lmao.

While a Hippo is 5'2 in height at the shoulders, they are about 11 feet long and weigh 4000 pounds. The average Asian elephant is 8'4 and weigh about 3200 pounds. Less than hippos, in terms of weight and muscle when compared to the hippo pound for pound. Which is probably why they're more tanky. Also elephants don't fight fully grown salt water crocodiles. 

 You should take you're own advice and "Might want to brush up on your animal facts." As you got everything more or less wrong. First if a rhino chooses to fight a Hippo it wins. Second Asian elephants are around 6,000 pounds (female), and 8,800 (male) and Hippos are around Africa Bush Elephants which are 6,600 (female) and 13,000 (male) so about twice to thrice the size of a hippo. Elephants don't fight fully grown salt water crocodiles? Do you mean Nile Crocs which are the ones that live with hippos and Elephants? In which case they do when attacked. Simply put Africa bush Elephants are the biggest baddest guys around over there and you clearly don't know that.

Quote from Milotroxisch >>

Why in the world is it so strong? I would understand it if it was a CR 2 or 3, but it's stronger that an elephant. Again I know that they are really scary irl, but not as scary as an elephant

 Because people don't understand animals which is demonstrated in all the replies to you. Also there are quite a few animals that make no sense. Three that I can think of are the Hippo, the Croc and the ape.

My only problem with this stat block is the speed. They should both be faster. Hippos, on land, are surprisingly fast for their bulkiness. As it is, according to the current stat block, a hippo and a typical human move just as fast on land. Real life, this is plainly not the case. As for swim speed, definitely should be more than 30 ft. I'd say it should be at least 40 feet for both land and water.

Hippos can run around 20 mph over short distance on land while some humans have ran in the high 20s. I think the speeds fine for them but i can also see it being 40ft on land. I don't think hippos are that fast in the water as they gallop at the bottom of the water which would actually mean they are slower then when they run on land.