They are spherical creatures that seem to take the role of dogs in the Mario universe. In Mario games they are often chained to poles and invincible. In Super Mario 64 there is a Chain Chomp in the first course.
In the blooper "Who let the chomp out?" it is revealed that one Chain Chomp is friends with Sonic the Hedgehog on Facebook. It is also revealed that their friend request was mutual. (If you don't know what that is, it is when both people send a friend request to each other simultaneously.)
His catchphrase “WTF is a Sonic?!” is from a now-deleted Jontron video.