About Tiger Claw's mutation
 

When Shredder first found out about mutant turtles, when he saw his henchmen mutate, when he saw an Utrom for the first time and when he found out Splinter had become a rat, he was (understanably so) confused. Each time.

At the start of season 2, he goes out to find Tiger Claw. Tiger Claw, we're told, is an infamous bounty hunter all across of asia. He's a giant tigerman who was transformed by Utrom in his childhood and spent his formative years as a circus act, then as an enforcer. He wields (I assume) alien weapons and isn't exactly unwilling to talk about his past. He also appears to be at least middle aged, meaning he's been around for decades.

How then can Shredder (and presumably hundreds or thousands of people) know about him, yet also also be puzzled by the existance of mutant animal-men and aliens?

Same reason Alopex was revealed to cut off Tiger Claw's tail despite the latter using male pronouns to describe the former this series, isn't good at planning ahead.

Oh yeah, hard to remember the pronoun used given that it was only said once...