They were pets.

Most likely Box turtles as the other common pet turtles are mostly aquatic and the TNMT were terrestrial.

Radical

Please refer to this paper on the subject. They’re actually four distinct species of turtle:

Kingdom: Teenage

Phylum: Mutant

Class: Ninja

Order: Turtle power

Family: Splinter

Genus: Pizza-eating

Species: Leader, Machines, Crude, and Party

These heroic half-shell types of turtles subsist mainly on pizza in the northern regions of New York, and travel in fearsome fighting teams, hunting Shredders and local Foot clan bottom feeders. They have evolved alongside the local giant Kung-Fu rat population with which they share a symbiotic relationship and a sewer apartment. When the evil Shredder attacks, these turtle boys “don’t cut him no slack” (as they’re known to let evil Shredders escape to provide weekly hunting experience and entertainment for the group).

If you’re lucky, you can catch glimpses of these elusive species on the local Channel 6 News with April O’Neil.

Red eared sliders, per the og comic, as far as I can find.

Why do you ask? Do you want to breed Teenage Mutant Ninja Turtles?

I disagree with the comment saying they were radical.

Its obvious they were bodacious.

My teenager offspring has extremely strong opinions on this and recently wrote an entire essay on how likely they are to actually enjoy pepperoni pizza given the original species involved.

But sadly, I don't remember the actual breeds involved, just that in the latest version they're all distinct species.

Teenage

What breed

Big ones.