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.
by DrZog February 24, 2011
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!
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