"I would not go that way if I were you. ...Peeves is planning an amusing joke on the next person to pass the bust of Paracelsus halfway down the corridor."
-- Nearly Headless Nick to Harry, near the Owlery (OP14)
Peeves the poltergeist was lurking in a hallway hoping to drop a bust of Paracelsus on someone’s head, according to Nearly Headless Nick who warned Harry about it (OP14).
When Newt Scamander realized that Frank the Thunderbird had not escaped from his suitcase, he said "Thank Paracelsus" (WFT).
Paracelsus was an alchemist and physician who experimented with potions.
Fan Theory: Cupboard Man
The bust of Paracelsus was connected to the Half-Blood Prince potions book, and helped Harry find the diadem of Ravenclaw:
In Harry's fifth year, he learned from Dobby about the Room of Requirement, and that Mr. Filch knew how to get in to find cleaning supplies (OP18). That same year, Peeves was messing around with the bust of Paracelsus trying to drop it on someone (OP14) and the bust in the "Sectumsempra" chapter of HBP is described as "chipped," so the poltergeist probably succeeded in breaking the statue.
In sixth year after using the "Sectumsempra" spell on Draco Malfoy, Harry entered the Room of Requirement to hide the Half-Blood Prince potion book from Snape. He passed the Vanishing Cabinet - also broken by Peeves near Filch's office (CS8), and where Montague was trapped by the Weasley Twins (OP28); later it was used by Draco Malfoy to smuggle Death Eaters into Hogwarts (HBP27).
Really this chapter could be called "A Tale of Two Cupboards."
Harry eventually found an old cupboard into which he placed the book. Then in order to find it again among the clutter of objects, he placed the ugly warlock bust on top, adorned by a wig and an old tiara. Some fans refer to this arrangement as the "Cupboard Man," with the cupboard being the body, the book as the heart inside, and the bust of the alchemist Paracelsus as the head.
"Seizing the chipped bust of an ugly old warlock from on top of a nearby crate, he stood it on top of the cupboard where the book was now hidden, perched a dusty old wig and a tarnished tiara on the statues head to make it more distinctive, then sprinted back through the alleyways of hidden junk as fast as he could go ..." (HBP24).
In some ways, Cupboard Man represents Severus Snape. Like the bust of Paracelsus, Snape is viewed almost universally as ugly. But of course Snape, like Paracelsus, was also a potioneer and healer, and the Half-Blood Prince book was inscribed with young Snape's improved potions. Snape was also associated with an ingredient cupboard in his classroom, and from which Harry, Hermione, Barty Crouch Jr. stole Polyjuice Potion ingredients, and Dobby stole gillyweed.Indeed, the cupboard in the Room of Requirement may actually be one of Snape's old cabinets that was damaged during a potions class gone wrong, since the door appeared to have been burned by acid.
When the Cupboard Man was first put forth, no one knew the tiara on top of the bust was also the lost diadem of Ravenclaw. Unfortunately, the diadem had been turned into an evil Horcrux, rather symbolic since brilliant Cupboard Man Snape has been corrupted by his life as one of Voldemort's Death Eaters.
During the Battle of Hogwarts, the Grey Lady's ghost told Harry that Voldemort hid the diadem of her mother, Rowena Ravenclaw, in the castle, and Harry suddenly remembered the bust of Paracelsus and the old tiara:
". . . the memory of a third stone effigy came back to him: that of an ugly old warlock, onto whose head Harry himself had placed a wig and a battered old tiara. The shock shot through Harry with the heat of firewhisky, and he nearly stumbled. He knew, at last, where the Horcrux sat waiting for him. . . ." (DH31)
The book, cupboard, and the bust of Paracelsus were all consumed by the Fiendfyre conjured by Slytherin Vincent Crabbe, who also perished in the flames (DH31). The diadem fell apart in Harry's hands soon afterward. And after Snape's death, nothing was left of Snape's legacy except in Harry's memories and the name he gave his son, Albus Severus.