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D&D 4th ed: The Warp Chicken

The Warp Chicken is the direct result of a player mocking a bunch of Dimensional Marauders (Monster Manual II, pg. 69) that were, admittedly, not powerful enough to seriously threaten his character. But still, in that one moment of good natured ribbing, he unwittingly inspired the horror which is the Warp Chicken.
While this monster is best played for laughs at first, it only takes one or two encounters with these things for the players to stop giggling and start taking them seriously, perhaps even acquiring a healthy dose of paranoia whenever the DM utters a few clucking sounds.

I was initially tempted to have some sort of clucking madness which these things could inflict on anyone repeatedly poisoned by their claw attacks, but decided against it.
You might be inclined to try it out.
I suggest using the Cackle Fever disease (Dungeon Master's Guide pg. 49) as a template, adding a few points to the difficulty class and changing all instances of the word Cackle, replacing it with Clucking.

Warp chickens appear in groups, looking a bit like featherless and mutated birds with slime dribbling suckers along their overly long necks and a mostly transparent, gelatinous looking body with twisted, unnatural organs and bones clearly visible through their pink and green veined flesh.
They are voracious feeders on organic matter, tearing apart any living thing they come across. Attracted to moving targets, the Warp Chickens have a tendency to mob and overwhelm any creature they find, followed by any plants in the area; right down to the dug up and devoured roots.
The wholly unnatural ability to rise from death and continue their frenzy of destruction is the most disturbing and deadly of the Warp Chicken's powers, along with the eldritch and creepy, gurgling clucking sound that they make when they do so.