Image Source: hippopotamuses.org
Really. Try it with me now.
(2) Two hippopotami have soiled the carpet in my den.
(1) I have a hippopotamus in my powder room.
(0) There aren't any hippopotamuses in the kitchen.
(0) On close inspection, I found zero hippopotami in the dishwasher.
When programming I have to test before composing sentences:
IF ABS(countVariable) = 1 THEN
textVariable = "hippopotamus"
textVariable = "hippopotamuses"
I know you're wondering. The preferred plural of hippopotamus is hippopotamuses, though hippopotami is accepted in the Oxford English Dictionary. Source: Wikipedia
Of course when encountering hippos in your dishwasher, I believe it would be correct to simply exclaim, "Snakes on a plane!"