Who invented Dragon Ball Z

Is Dragon Ball Z an invention? No.

Toei Doga created it.

there's a picture in the credits that says Toei animation with that drawing of a cat with a feather in its hat at the end of the show.
They did the anime, but the story/manga was created and drawn by Akira Toriyama, who has also done Dr. Slump and I think Hunter x Hunter. Toei took the original illustrations from the manga and animated it. Akira Toryama is the father of the original though. So he gets pretty much all of the credit.