A group of hippos is called a herd, bloat, or crash.
hippos are indeed mammals ;)
A bloat of hippoes
Also, Pod, Herd or Dale are often used
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Hippos belong to a group of animals called pachyderms,which include elephants, rhinoceroses, tapirs, and ironically: manatees and dugongs.
The collective term for a group of hippos is a bloat, herd, pod, or school.
See http:/www.askoxford.com/asktheexperts/collective/h/?view=uk