Gulpo is a giant blue piranha Nightmaren in Nights into Dreams thats inside a cylinder based tank. He is the boss of Mystic Forest. To defeat him you must fly to the nearest fish thats facing directly at Gulpo. You will find your self blasted through the tank and hit Gulpo. Repeat 3-4 times to defeat him. But you must be careful not to be blasted into his mouth. He can also attack by sending purple orbs into the fish around the tank, making them gain a dark purple aura which will hurt NiGHTS on contact.
A translated Japanese biography for Gulpo states:
"This fish-type Second-Level will swallow anything up completely with his huge mouth. His form is ferocious, but his personality is surprisingly calm. Knowing his weakness is his slowness, he’s set up traps all around his body to catch his prey."
Gulpo appears in the Nightmare Zone DLC for Sonic Lost World, as the Nightmaren that Master Zik is riding on.