Malthus is the main secondary antagonist in Miss Peregrine's Home for Peculiar Children. He is a Hollowgast, a former ymbryne turned into a monster which possesses drastic abilities.
In the 1900s, some ymbrynes (advanced Peculiars with paranormal abilities including the ability to control time) grew impatient at their lurking in the time loops when they could subjugate humanity. Thus, they performed an experiment in the remote wild of Siberia, and this backfired into a massive explosion which destroyed the woodland and mutated the ymbrynes into evil monsters which now sought to conquer the world and kill humanity.
Before the novel
Malthus is the primary antagonist for the first half of the novel. He stalks Abraham, Jacob's grandfather, and threatens his life because he is a peculiar (although Jacob doesn't know anything about his grandfather save that he hunted monsters in Poland in the 1940s and Jacob took this to mean Nazis) and eventually, before Abraham can properly warn Jacob of the danger, Malthus attacks and viciously kills Abraham. Jacob, en route to the house, sees a creepy white-eyed old man on a lawn, but he dismisses it as a regular blind human.
Malthus remains to oversee the murder and when Abraham tells Jacob of "The Bird" he then dies and Malthus reveals himself to Jacob, shocking him so much he is put into care - with the ironic twist his therapist is Dr Golan.
During the novel
Malthus returns when Jacob has been granted a holiday by Dr Golan to go to Wales, and he quickly finds the locals pretty stuck up and hostile. Then things take a turn for the peculiar when he walks into the bombed out Miss Peregrine's Home for Peculiar Children. Before he arrived Jacob was a rationalist and believed the children were dangerous, or were mentally unwell, thus "peculiar" however he becomes a believer in magic when he steps back in time to the 1940s, and meets the children in the flesh, being rescued by a young girl named Emma who he grows fond of.
The children - a flying girl named Olive, a pryomaniac named Emma, an invisible boy named Millard - take Jacob to meet their headmistress, Miss Peregrine, who tells Jacob of his grandfather's true history. She says he was a peculiar and left to hunt monsters, but she doesn't say what monsters yet.
Jacob stays for dinner then returns home to his own time the next day.
Later in the novel, a flock of sheep are slaughtered viciously by Malthus, yet the blame is put on some rowdy youths in the island. Next, there is a murder there - the local historian, Martin, is slaughtered by Malthus, mistakenly thinking it is his prized human mummy which attacked him in his final moments. Malthus then sends Dr Golan to the island, posing as a birdwatcher, who grows to irritate Jacob's avian-obsessed father.
Miss Peregrine reveals to Jacob the history of the Hollowgast, former ymbrynes who turned into evil monsters because of their lust for immortality and power. Jacob begins to suspect Malthus of being a Hollow, and then Emma reveals that Jacob is also a Peculiar, because he can see the monsters no one else can and that Peculiars are the only ones to go through time loops.
When another school for peculiar children is attacked, its children devoured by Malthus and its principal fleeing in terror to Miss Peregrine, Emma, Olive and Millard go with Jacob to reanimate the body of Martin in order to see who killed him. They find out that Malthus was responsible but Martin still thinks it was the mummy who killed him. Malthus returns with Dr Golan and Golan reveals himself as a wight (an advanced Hollowgast) and unleashes Malthus on them.
Emma and Jacob send the others back and lure Malthus onto the moorland, where he slaughters sheep in an effort to get them. Malthus almost kills Jacob with his tentacles, and pulls them down into a bog, but Emma yanks Jacob out, and Malthus is left to slowly sink to his doom after Jacob stabs him multiple times in the throat.