According to the writings in The Lesser Key of Solomon, Forneus is the thirtieth spirit listed in the Goetia. He is a Great Marquis of Hell with twenty-nine legions under his command, partly comprised of the order of Angels and Thrones, and appears as a sea monster. When summoned, he can make men well-versed in rhetoric, give him a good name, teach him foreign tongues, and make him trusted by friend and foe alike.
Forneus is a boss found in the Hospital which you have to fight against. He doesn't like you and won't let you by. Upon defeating him, you can leave the hospital. Forneus is suggested to be a friend of Decarabia, although there is no given explanation for their friendship. Some suggest that it is because both appear as creatures of the sea, others that it is simply because they are both Marquis of Hell, although this is still only conjecture.
Shin Megami Tensei IMAGINE
Forneus can be fused from a Divine Angel and a Wilder Bicorn or a Dis and an Inugami.
Devil Summoner: Soul Hackers
Forneus appears in the 3DS remake as a demon unlocked with Nemechi's cat form, for 100 D-Souls. He is the earliest demon with access to Makakaja.
Persona 3
The special non-mythological relationship with Decarabia is rendered again in this game, in which these two personas are able to perform a combo skill called Best Friend.
Digital Devil Saga: Avatar Tuner
Forneus will appear after defeating lone Isoras in the northern tunnels of the Samsara Tunnels.
Requests Macca. After using on the same clan of demons thrice, any demon of that clan will take away one third of player's Macca forcefully in the next encounter.
Forneus as he appears in Shin Megami Tensei III: Nocturne.
Forneus in Majin Tensei
Trivia
Kazuma Kaneko made his design look like a manta ray because the manta ray are sometimes known as "devil fish"; Forneus appears as a sea monster. It also appears to have a human body merged into its back.
