From Muppet Wiki

Donny Dodo is the son of the Dodos (Mommy and Daddy) in Follow That Bird.

As Big Bird's temporary foster brother, the nasal voiced, slightly smart-alecky Donny spends more time squabbling with his sister, Marie Dodo. Neither Donny nor Marie display any capacity for imagination.

Donny and Marie's names are a reference to the pop duo Donny and Marie Osmond.

Donny also appeared in The Muppets: A Celebration of 30 Years.