Because they don't swim in pairs. Are there two of those fish? Nahhh.
A starfish !
Minnow-pause
Aardvarks don't have trunks!
A bulldozer
A pair of ankles
Double D-Mons
They keep getting caught in the net!
You can tuna piano, but you can't piano tuna. ... What about the pot of glue I knew you'd get stuck.