Illudwaas

An artificial fish with a rune on their back and a ghost-like appearance.
They are believed to have been created by an alchemist, though for what reason is unknown. 
This incredible (and infamous) fish is known to teleport away from fishermen after being caught. The only way to prevent this from happening is to fully cover the rune. (Note: No you can't put them in your mouth for this, I'm looking at you Maxie.)
It's 9 in. (23 cm.) long. They have thin yet strong skin, the colors of which are usually pale greens, pale blues, purples or various browns.
 
They live in large rivers and ponds. They are common. They're omnivores and their fairly stretchy mouths and long tongues are ideal for eating all types of aquatic life.
They're crepuscular and rely on their hearing and sight to maneuver.
They have large, glowing elliptic eyes and tiny, hidden ears. Their heads are evenly sized in comparison to their bodies.
 
They make sounds ranging from high pitched to low pitched and have a large range of sounds they make to communicate with each other.
These creatures are quite frenzied, but they're not one to defend their territory. They mate twice a year with a select few partners during life. Which, with their fairly long lifespans (20-36 years), is to be expected.
 
It's not recommended to eat this fish, but those who have describe the taste as tough, hard to chew, dry and bitter.