The Fifth Turtle

There was indeed a fifth Ninja Turtle and his name is not Venus.
Well, technically Venus still is the fifth turtle from the short lived Ninja Turtles : The Next Mutation.

Video Evidence:

But not so recent searches would suggest a fifth turtle supposedly part of the unproduced IV live action TMNT movie from the 90's.
A turtle that would not be named after a renaissance artist or a work of art but rather after the King Of Comics himself, Jack Kirby.

All artworks as drawn by Kevin Eastman back in 1994-1996.

Further analysis would suggest that this concept and design is what would later on be The Mighty Mutanimal Slash.

But Slash would predate this concept art further debunking this theory.

Video Evidences: