Negasus can be found on the 1st and 2nd floors of the Divine Paradise. He can also be found between 5000 and 10000m into the Infinite Tunnel. He can also be found disguised as a trash can through the alleys in Blossom Heights.
Lastly, he can be freed with a Blue Coin from the past Crank-a-kai.
Negasus appears in the anime in Yo-kai Negasus, where he causes Nate and Katie to commit some mischief at school.
Negasus later appears in EP142, where Hailey 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 Hailey 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 Phonius into thinking that they're the real deal.
Etymology
His Japanese name, "Magasasu," is a wordplay of "ma" (魔, evil spirit) and "Pegasus" (ペガサス, Pegasasu).
His English name, "Negasus," is a wordplay of the words "negative" and "Pegasus." It may also be a pun on the word "nemesis."
Trivia
Shortly after being inspirited by Negasus, Nate does a pose with Negasus in the background, which is a nod to Seiya from Saint Seiya.
In the original 1.0 version of the US and Australian release, Negasus Soultimate was named Negasus Shock.