The "Skeletons" are characters that first appeared in one of the classic Silly Symphonies, specifically the first entry in that series, The Skeleton Dance, which was released in 1929. The skeletons are obviously undead walking "creepers" and have been long associated with the macabre side of Disney, though they are not actually malevolent in most of their appearances, resembling harmless outgoing pranksters.
The Skeletons are the main protagonists of the short. The first one rises from behind a grave and scares away two cats and arranging itself into the classic skull and crossbones formation, before leaping into the center of the graveyard and being joined by three more. They proceed to dance and frolic in the graveyard, in general good-hearted dances and musical games, though they are somewhat reckless and unintentionally harmful in their behavior (A skeleton threw his skull at an owl in retaliation from being scared from its hooting and knocked off its feathers, they hurt a cat when attempting to use his whiskers like a violin, and one of them takes the femurs of another then using him as a xylophone until he collapses). When the rooster cries, the Skeletons panic (probably because, as in common ghost lore, they cannot stand sunlight) and merge into a single several-limbed creature that hides in a large tomb.
When Mickey Mouse enters a haunted house, he is repeatedly scared by the Skeletons (who are more numerous than in their debut). He eventually comes across their leader, a hooded skeleton with a missing tooth (sometimes thought to be the Grim Reaper, though he isn't identified as such). Surprisingly, as he is cornered, Mickey is simply told to play the piano for them, and all the Skeletons begin dancing and frolicking in the haunted halls, much like in The Skeleton Dance. Mickey, still scared, tries to escape the house, pursued by several Skeletons (and coming across more of them).
The Mad Doctor's large castle is haunted by several Skeletons. Most of them are hiding in the furniture, including in the lift, cupboard, inside a stone pillar and inside the steps of a staircase (which are apparently really coffins). They attack Mickey, though whether they inhabit the Caslte and have decided to get rid of trespasser, or are doing so on the Mad Doctor's orders, is left unclear. A skeleton spider (an obvious nonsensicality, seeing how spiders are invertebrates, which could however be explained by the Spider being a creation of the Doctor's) also appears and attempts to eat Mickey.
Some Skeletons are seen in the projector reel transition level based on the "The Mad Doctor" cartoon. Hidden-unseen skeletons eerily lift up the stairs that can make Mickey fall down and another towards the end will throw a piece of brick at you and will cause you to lose some health. Some others appear in the second and third transition levels themed to "The Haunted House".
In Lonesome Manor, Mickey confronts minions of the Mad Doctor who resemble Bashers but, instead of being those of pirates, their toon parts look like the skeletons. Since the skeletons were already the Mad Doctor's minions in his debut cartoon, it's possible he turned them into Bashers just like he did the pirates of Tortooga.