Yes, I know this technically neither "animals" nor "de-aged" as the Theme Number 25 called for, but I couldn't stop thinking of Oougel, the escaped product of the bad guys' mad science, as Mewtwo. C'mon, man, they're the same freakin' character.
As for the rest, we have Hour Thief as Haunter because stealing time is creepy, Zarp as Charizard because dragons, Timesplice as Celebi for fairly obvious reasons, Muppy as Aggron the resident really big smashy guy, Kitai as the ever-effective Ninjask, and Psi-Dome as Metagross because it has four silicon-based brains that are powerful enough to warp reality. And it can hover, too.