Some TMNT stuff was always meant for grownups.

Marlin's Cryptids are a gathering of creatures that are commonly thought to be only present in myths and legends, that were captured by Mr. Marlin. They appear in the Mirage Comics issues World's Deadliest and Monster Island.

The collective of the cryptids included the Jersey Devil, Mothman, Bunyips, Sasquatch, and the Loch Ness Monster.