Aliens

Tretinleius Maximus Tertius
These were microscopic tregotolotoids from Zudon-9. One of their tiny spaceships once got into Chuck Jr.'s nose and gave him a cold.

Dingons
Hailing from Dedingos, these brown aliens communicated in "dingaling". They were short and stocky, with coral spots on the outer side of each eye. The eldest males had curly horns. They once crash-landed in a tree, but were found by Ike, Joe, and Slithers.

kHns
kHns were even smaller than Tretinleius, and preferred water. They lived in swirling schools of trillions at once, and hailed from the giant water planet of eTo. They ate anything, and almost did so to Joe, Ike, and Nutty when the Adventure II crashed.

Aliens in the galaxy
Apart from Chuck Jr, Jay, Ike, Joe, Slithers, and Nutty, few Earthlings knew of other life. It is known that by the end of the extragalactic Sagittarius War, Nutty Jr had contact with at least 7 advanced civilizations. Nutty's Lab hosted an alien-communicator and dimensional portal, both of which could be used to contact other planets or universes. It is thought that in the solar system, there were 2 civilizations; approximately 500,000 in the Milky Way; 6,000,000 in the Local Group, and about 500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 in the universe. A total for the entire multiverse would be approximately 4 googolplex.