They form an extravagant and obscenely wealthy (yet often clueless) hippo couple who live in the jungle yet speak with Barcelonian accents. Flavio, the husband, was voiced similarly to Ricardo Montalban. Bored of their pastoral life, they came to the big city and settled atop a skyscraper in a lavish penthouse suite, according to the theme song.
In the series, an overexuberant naturalist named Gina Embryo strove to keep the Hip Hippos from moving to the city by masquerading as a real estate agent and showing them undesirable location after undesirable location...but they liked them all. Geena would incur injury after injury in her quest to preserve and observe the magnificent beasts in both the shorts she costarred in.
At other times, the Hippos parodied the opera La bohème (as "La Behemoth"), the shirtless hunk advertisement for Coca-Cola, and the show American Gladiators. Brain even attempted once to use the Hippos in his scheme to rule the world by masquerading as their baby. (He was actually masquerading as the Rockefeller family's baby; the stork goofed up.)
The pair had relatively few starring roles, but were frequent guests on other shorts, often fulfilling some form of sight gag poking fun at their immense nature (such as stepping on smaller characters). They did not even appear in the Animaniacs comic book until issue #38.
List of Sketches
A Moving Experience
Can't Buy A Thrill