It was Drooper, the lion. Every so often, Fleagle (the dog, voiced by the legendary Paul Winchell) would say, "Hey Drooper... take out the trash!" And then Drooper would have to deal with whatever was in the trash can waiting for him.

Likewise, they'd make Snorky the Baby Elephant "Go get the mail" out of the mailbox. And, also likewise, there would be surprises awaiting him inside that mail box.
If you review the opening/closing theme video above, it will reference snippets of Drooper's encounters with the trash can.