Brain Fish are a species in Metaworld.
Jon Ulmulk used his evolution science to create the Brain Fish, an alien brain which turns violent beasts into loyal soldiers.
The Brain Fish are essentially insect, which have red eyes, two spikes, and innards. They come in the colors orange, yellow, red, or one of the two shades of blue and green. The brains have three stubby legs on each side of their bodies, spiked tails and small jaws.
The top of the Brains resemble a real human brain and atop each Brain are two red spikes. Their eyes are glowing red with small, sharp, white teeth. Each Brain has three legs on each side and six legs altogether which are extremely insect esque. The fact that the Brains have six legs supports the theory that they are a cross between an insect and a brain. They have tails with spikes and can change color once they have attached themselves to a victim.
A Brain is a lab-spawned insect with the ability to take control of the minds of aliens and act as the host's brain. Since the Meta Monsters are said to be low in intelligence, the Brains can take complete control of it and act as the host's brain.
When a Brain attaches itself to a life form, it causes the life form to become fearsome in appearance and since the Brain's eyes replace the life forms' normal eyes, they appear glow red. The life form also mutates and puts on more muscle mass while also growing new appendages and organs to better suit the new creature's needs.
All of the Brains are spawned in a place known as the Brain Factory by Jon Ulmulk who launches them throughout the Earth in small vehicles that resemble a cross between a test tube and a star ship.