Negasus can be found on the 1st floor of the Divine Paradise. He can also be found 5,000 yds into the infinite tunnel.
Negasus appears in the anime in EP068, where he causes Nate (and Katie) to commit some mischief. At home Nate attempts to smell his dad's sock, but this proves to be too much for both him and the pegasus Yo-kai.
Negasus later appears in EP142, where Inaho uses him as part of a phony Jersey Devil. USApyon suggests Negasus as a horse-like yo-kai to use for the equine-esque cryptid, but Inaho realizes that Negasus's feathered wings don't match the Jersey Devil's more bat-like ones. Luckily, Hidabat volunteers to ride on Negasus's back, providing the illusion of bat wings. After spray-painting Negasus black and giving him fake claws, the Negasus/Hiadabat duo is able to fool Yarase into thinking that they're the real deal.
Shortly after being inspirited by Negasus, Nate does a pose with Negasus at the background, which is a nod to Seiya from Saint Seiya.
"Negasus" is a wordplay of the words "negative" and "Pegasus." It may also be a pun on the word "nemesis".
"Magasasu" is a wordplay of "ma (魔, evil spirit)" and "Pegasasu (ペガサス, Pegasus)."