In the earliest shorts, Cosmo had a deep, suave voice, and was less moronic, and he and Wanda were both relatively careless and bubbleheaded ("We're two halves of a whole idiot!"). This as since changed, as over the course of the show Cosmo's changed from one half of a fairly goofy couple to the main source of carelessness and trouble, while Wanda's changed into a more sensible character.
He once bet Juandissimo Magnifico to Wanda for a nickel, which he apparently likes because of its luster. In another episode, he finds a nickel and names it Phillip. Phillip is a girl nickel (as noted in the quotes section below), and is seen in at least one other episode.
There's been several instances he has no brain, but somewhere he has a really tiny one, as shown in the first power hour. But his head is also full of grape jelly, dancer was dancing on his brain one time, and sometimes, elevator music can be heard playing from his head.
He's got a pet hog named Steve that lives in his pants.
In the original cartoon, featured in 'Oh Yeah! Cartoons', Cosmo was actually smart.
Several instances (his name, his name in Timmy the Barbarian) refer to the name Julius.
His name in Greek means "world".
He supposedly wears a diaper. He has been seen wearing underwear most of the time, but after Fairly Odd Baby, he was seen wearing diapers throughout the show.
All of Cosmo's pets are named from Jimmy Neutron Characters (Carl the Ant, Cindy the Bee, Jimmy the Cat).
He often flirts with other female characters he deems attractive (e.g. Tooth Fairy) which often angers Wanda. He has also fallen for Princess Mandie, Blonda Fairywinkle, his reflection, a doll that was later transformed into Tootie, & various others.
He likes rice, pie, corn, lemonade, pudding and churros.
His favorite flower is the rose as revealed in the video game "Shadow Showdown".
Cosmo's old band is called The Fairies. It consists of himself on the electric triangle, Eric Clapton on the guitar, Pan on the pan flute, Santa Claus on drums, and Ludwig van Beethoven on the organs.(They were seen in Boys in the Band)
Cosmo is allergic to nuts, revealed in the "Squirrely Puffs" episode, although in past episodes he has been seen eating nuts without allergic reactions. The nuts he ate in Squirrelly Puffs may have been a special kind of nuts that he is allergic to or that Cosmo takes a medication to prevent having an allergic reaction to nuts.
In the anime series Sonic X, there is a character known as Cosmo in its third season. While both characters have the same name and color of hair, the only 2 differences is that they have opposite genders and different eye colors.
He is apparently the reason why Denzel Crocker lost both him and Wanda due to allowing himself to be revealed to humans.