Bud and Lou

Bud and Lou are fictional characters, the pet hyenas of The Joker on the cartoon series Krypto the Superdog, in which the Joker himself never appeared. They are primarily the foes of Ace the Bat-Hound, the pet of Batman, and were first introduced in the episode "The Dark Hound Strikes!" They attempt to commit crimes for their master, requiring Ace and Krypto (and Streaky the Supercat, on occasion) to stop them. Their names are references to the comedy duo Abbott and Costello.

The Joker's hyenas have also appeared in Batman: The Animated Series, The Batman and the comics, but remained unnamed or simply referred to as "Babies" by Harley Quinn. Joker mentioned their names as Bud and Lou however in The New Batman Adventures episode "Joker's Millions" (which aired before Krypto the Superdog).

They both also appear in Batman: Arkham City where The Penguin has them killed and stuffed in his museum as an exhibit prior to the events of the game.

Characteristics
In Krypto the Superdog both hyenas have red fur with black spots and black rings around their eyes. This may be a reference to Harley Quinn, the Joker's girlfriend and accomplice and their mistress. Bud wears a purple collar, has a black right front paw, and speaks in a higher voice. Lou wears a green collar, has a black left front paw and right hind paw, and speaks in a more gravelly voice. (However, in Batman: the Animated Series they are identical and both wear red collars (purple in the episode "The Man who killed Batman"); in The Batman, they are identical, wear no collars, and are grayish in color.) Both hyenas have a distinctive odor. This was first mentioned by Ace in the episode "The Cat and the Bat". Later on, in the episode "Funny Business", Ace told Krypto he could smell their evil.

Like their master, Bud and Lou love to laugh maniacally and play jokes on their foes. Like the other animal characters on Krypto the Superdog, they also seem to be quite intelligent (a reference to the intelligence of actual hyenas). For instance, in "Bat Hound and the Robin", while stealing pillowcases for The Joker to carry loot in, they deduced that the pillowcases made of one material would be better because pillowcases made from the other fabric would shrink in the wash, meaning less loot could be put in them. Also, The Joker has given the hyenas items to use in combating Bat Hound. At one point in "The Dark Hound Strikes!", Bud pretended to give himself up. When Ace seized Bud's collar in his fangs, he received an electric shock. Bud then announced he had a joy-buzzer collar and laughed. In "Funny Business", the hyenas had a bubble shooter which shot special "giggle bubbles". These bubbles caused Ace to laugh uncontrollably, a mild version of The Joker's laughing venom.