With the help of the player, Dogday could finally hit Catnap with an antidote, getting rid of that horrible body. Right after getting his legs back he went straight to the now harmless cat, but instead of killing him, he wanted to take revenge for all the suffering he had to deal with thanks to his purple ex-buddy.
Nonstop sex for all eternity sounded like the best way to relieve all his stress.
As for Catnap, he couldn't do anything but watch and feel all that accumulated pain of so many years from Dogday expanding his ass and going right through his stomach.
Maybe one day they'll resolve their differences and be friends again.
...
that was the secret ending of Poppy Playtime... with pubes