They are a very strange race that desires everything humans hate; they consider garbage to be food, beautiful to be hideous, and insults to be compliments. The most notable Yugopotamian character is Mark Chang, the Prince of Yugopotamia and son of Queen Jipjorrulac and King Gripullon, who later moved to planet Earth in the episode "New Squid In Town!".
The Yugopotamians are squid-like creatures with tentacles and exposed brains encased in glass. They range from several shades of blue to purple, while a few are colored light green and aqua green. Sweet things like candy can burn through them like acid. Some Yugopotamians wear fake-i-fiers, pink and purple devices with a spinning six-spoke star-like symbol similar to an iPod's touchwheel (a variation of this device was even called an iFake), which allows the wearer to disguise themselves as humans and other Earth objects.
The Yugopotamians also have a bomb for an appendix which detonates if they do not celebrate the five days of F.L.A.R.G., a series of events that are viewed as disgusting or miserable to the average human being but are very sensational to the Yugopotamians. Apparently, a Yugopotamian's appendix is able to detect the excitement of the celebration, which then defuses the bomb.
They are shown to be somewhat violent and panicky, if not moronic. When King Gripullon was spying on Earth during Halloween, he thought the stockpiling of Yugopotamian-lethal candy was an act of war. He tried to attack Earth with a bomb made of candy. Mark Chang acts more like a human teenager, considering his trips to Earth to be "road trips" and speaking in a surfer-dude tone. Mark Chang is hopelessly in love with Vicky the Babysitter, her cruel behavior only causing him more infatuation toward her.
In most episodes they appear in, the Yugopotamians show their fear towards nice and cute things. Whenever they become extremely frightened or overwhelmed, the glass dome encasing their brain can explode and shatter. The glass dome can also be lifted as it seems to be attached and hinged to the Yugopotamian's head, allowing him to access a compartment in his head where he can keep things. This is seen in the episode Five Days for F.L.A.R.G., where Mark Chang lifts his dome and reaches in to retrieve his F.L.A.R.G. manual.
Yugopotamians usually eat trash. They eat desert first. Their deserts consist of broccoli and brussel sprout sundaes. Chocolate and other sugary things would not settle well with Yugopotamians. The tastiest of all Yugopotamian deserts is Spinach Cobbler.
The Yugopotamians, mainly Mark, his parents, and Mark's friends Jeff and Erik, have appeared in several episodes. In a few episodes, such as "Scary Godparents", "New Squid In Town!", and "Planet Poof", they seem to be semi-hostile toward Earth, and they hoped to form an alliance with the Boudacians by forcing a marriage between Mark Chang and Princess Mandie so that they could attack and destroy Earth together with their combined forces. However, Princess Mandie's intent was revealed in "King Chang" to have been to conquer both Yugopotamia and Earth.
In the episode "So Totally Spaced Out", the Yugopotamians were easily conquered by The Gigglepies, a race of merchandising Carebear/Bunny creatures whose nice and sweetness brought the Yugopotamians to their knees... until they learned the Gigglepies tasted like manure (a delicacy to them), which resulted in a feeding frenzy and the Gigglepies being driven off their planet.
At the end of Force of Nature, the Yugopotamians are present, eating all the extra garbage left by the people of Dimmsdale, and King Gripullon says that they have to come back to Earth every Litter Day Weekend in order to feast on their garbage.