Doraemon: Nobita in the Wan-Nyan Spacetime Odyssey (映画ドラえもん のび太のワンニャン時空伝) 20 years old

An elderly dog goes to a time machine, which he plans to use to drive to the future so he can meet with "someone" who gives him a kendama. However, the machine goes haywire and the dog is regressed back to his infant state, eventually found by a researcher cat.