Donkey Senior was Donkey's great-grandfather. He was shown in the "Swamp Talk" episode, "Donkey's Awkward Family Photos".
While only shown in a sepia photo, Donkey mentioned that he cured "Magic Measles Disease".
Being from the show "Swamp Talk", which has already shown a lot of inconsistencies with the rest of the franchise, it's safe to say that Donkey Senior is not a canon character.