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Fat creature which is ordinarily tame but occasionally bursts into psychotic rages for no apparent reason.
Clyde: What the hell happened to you?!

Zeke: Good god, all I said was I didn't like eating at that new restaurant and she turned into a friggin' psychopotamus!
by Krakky McKraken November 12, 2006
When someone is a huge psycho. It is a cross between a psycho and a hippopotamus. Hippos are large animals and thus psychopotamus' are huge psychos.
Claribel won't leave me alone, she's a psychopotamus.
by DrZog February 24, 2011