Here are some options you could use, though:
* A bloat of hippos
* A herd of hippos (this is the most common, though not technically accurate, as hippos don't form herds like cattle)
* A pod of hippos (this is sometimes used as hippos can be found in groups, but it's more commonly used for whales)
* A wallow of hippos (this is a bit whimsical, but it works as they are known for wallowing in mud)
Ultimately, you can use any collective noun you think is appropriate for your context!