This is gangster lingo mockery.
What's up with them all calling eachother G-Dawg, and stuff, my friend and I thought?!
I mean, why be so fcuking MEAN! the other letters and animals SURELY DESERVE A CHANCE!
So, y-fish, like any letter put with any animal, is a gangster nickname. Like homee.

Y-Fish is also, as it happens, my stage name, with P-Hamsta, we wish for PETROL PRICES TO GO LOW! and specialize in parodies.