Appearing in the pilot episode "Woo-oo!," the Headless Man-Horse was initially dormant within his saddle at McDuck Manor until he was summoned when Louie unknowingly sat on the saddle. He then teamed up with Captain Peghook as his steed with the Deus Excalibur as their weapon against the children, until Scrooge McDuck shows up to take them head-on. Scrooge exploits Peghook's demand for his head by tricking the captain into chopping off a statue's head, causing the captain to disappear, while the Headless Man-Horse receives the head. Grateful that he has a head, he thanks Scrooge in Morse code and leaves the manor.
In "The Shadow War!", he aids in fighting the shadow army under threat of being fired. He and Lil Bulb are surrounded by shadows and are saved by Fenton.
In "The 87 Cent Solution!", he appears in a casket pretending to be Scrooge with his bust painted so that it would look like the real Scrooge in order to trick Flintheart Glomgold into thinking that Scrooge is dead so he can get Glomgold to confess to his crime.
In "Moonvasion!" he participates in the fight against the Moonlanders.
In "Louie's Eleven!" he participates in Louie's scheme (#11: The Muscle). When the plan gets conflicted with Falcon Graves' own heist, Manny briefly engages him until Falco tosses him aside, causing his head to end up on the body of a statue of Hyacinth Hippo.
In "The Last Adventure!", he is revealed by Gene the Genie to be "The Headless Horseman of the Apocalypse". During the episode, his statue head is given life by the Phantom Blot using the Blessed Bagpipes, granting him the ability to speak (though it also petrified the rest of his body to trap him). Later, Lena and Violet Sabrewing arrived to prevent the Blot from draining his magic, unpetrifying him freeing Gene in the process. Hoping to help Lena fight the Blot, he regrew his real head and sprouted gargoyle wings, becoming a horse/gargoyle hybrid (which he "swore to never [do] again" at some point). However, he fails and they get captured by F.O.W.L. and the mind-controlled villains alongside Violet. They are all later accidentally freed by Launchpad McQuack wearing the GizmoDuck armor.
Manny appears in issue #8 of the series. Taking place before his appearance in "Beware the B.U.D.D.Y. System!", Scrooge hires Manny as his fill-in for advisory business meetings after Manny was declared redeemed by Scrooge. Scrooge intended to have his absence re-affirm his authority over his advisors, but the advisors take a liking to Manny. However, Manny ends up indulging in Scrooge's money and resources behind his back. Scrooge confronts the horse and has to fire him after Manny puts up a fight until Bradford Buzzard knocks Manny out. Afterwards, Manny is sent back to the Rehabilitation program.
Manny's name was first revealed in the show's official comic.
He communicates by Morse code.
It appears he breathes through the Scrooge McDuck bust as in The 87 Cent Solution, he covers the mouth with a germ mask.
Manny's role in the series finale wraps up the show's Disney Afternoon crossover theming through multiple references to Gargoyles, with the show's theme song playing when his head and wings grow back and paraphrasing Goliath's introductory narration by declaring "I live again".
He closely resembles the London Clan of gargoyles, as a humanoid body with animal features and wings.
Him being the "headless man-horse of the apocalypse" is a nod to the television series Sleepy Hollow where the headless horseman is also Death, one of the four horsemen of the apocalypse.
According to Phantom Blot, the headless man-horse of the apocalypse is fated to bring about the end of days. But Manny say's he's changed and just wants a day job.
Manny actually has a secret Twitter account, maintained by Frank Angones, co-executive producer and story editor of the show: @HeadlssManHorse.
Manny ecstatic that he finally got a head (a bust of Scrooge)
Scrooge leads the charge on Manny's back (to his chagrin)
Manny's Scrooge statue head finally talks in the series finale
Manny grows his head back and wings in the series finale