A Myrshnik is an Arcadian creature. In Chapter 8 of Dreamfall, Crow tells Zoë not to fear that the smoke bomb may harm the guard in Friar's Keep. "He might wake up with a headache. Just remember that he would cut your head off without a moment's hesitation. It's a myrshnik-eat-myrshnik world, baby. And only the strongest myrshnik survive." The resemblance to the real-world saying "It's a dog-eat-dog world" implies that a Myrshnik is a dog-like creature.
The real-world aphorism takes its modern, English form from an inversion of that attested in the 19 June 1789 issue of the London Times, which read "it is an established fact, that sharper will not rob sharper, nor dog eat dog." This came from the old Latin proverb Canis caninam non est ('dog will not eat dog'), known from Varro's Lingua Latina VII. Evidently, a world in which dog will eat dog is in trouble.
Ragnar commented on the Myrshnik on his blog (web archive) in response to a reader's email, confirming that it was indeed a creature. He also promised in a Reddit AMA answer that "we will see something of the Myrshnyk in Chapters".