YoUDee (pronounced yoo-dee) is a mascot of the University of Delaware, along with Baby Blue. It is an anthropomorphic "fighting Blue Hen", and its species is the blue hen, as the state bird of Delaware. While YoUDee appears masculine and can have male or female performers, it is "officially androgynous", "neither female or male".

According to the University of Delaware, YoUDee's colors are Blue and Gold because its great-great-great-grandfather was awarded the gold medal for Valor during the Battle of Trenton in the American Revolutionary War. The men in the company commanded by Captain Jonathan Caldwell actually carried Blue Hens with them into combat in order to hold cockfights in their spare time. It is said that the men under Capt. Caldwell had bravery in battles that was comparable to their fighting blue hen chickens.

In 1911, the University of Delaware adopted the Blue Hen as its official mascot.

In the 1930s, the initial physical mascot was a live blue chicken that was tended to by members of the cheerleading team. It is unclear if it was a blue hen, or a chicken with blue feathers, as it is believed there are no true descendants of "fighting blue hens of Revolutionary War times."

Starting in 1950, several costumed "Fightin' Blue Hen" mascots existed. Little is known about them besides the fact that they were "a variety of light and dark blue suits." Few photographs of them can be found. All of the costumes were presumably retired before YoUDee appeared.

One reader of The News Journal in Wilmington requested that one of the old mascot's costume should be displayed in the nearby Bob Carpenter Center. This reader was not alone, as a petition with over 1,000 signatures circulated the state of Delaware, demanding the same. The Athletics Department responded and objected to the petition on the grounds that there was no room for it, and that "it would be inappropriate." Articles from the time indicated that at football games, the old mascots were ridiculed and pelted with items such as "marshmallows" and "batteries" by students on a frequent basis. It is unclear if this was a recurring skit coordinated by the mascot team, or represented student emotions at the time.

A successor mascot would take its place. When the University of Delaware President David Roselle was questioned as to why there would be a new mascot, he said "[well], the obvious answer is, the old one wore out."

YoUDee was first seen (or "hatched") at the Delaware-Lehigh football game in Delaware Stadium on September 4, 1993, after being created by Tom Sapp of Real Characters, Inc. (an Atlanta-based advertising agency.) Student performers were scouted to wear the costume. After two students applied and performed in tryouts, Robert Boudwin became the first performer of YoUDee. Boudwin worked to build YoUDee's personality and far-reaching image from "Delaware's infamous mascot" to "ass kickin' chickens." With help from David Raymond, the original Phillie Phanatic, the university mascot program has become a gateway to several professional mascot careers.