Consider, if you will, the OBB

First, why did they always capitalize the monsters in the original Gamma World book?  This is 1st edition Gamma World, I never got my greasy little digits on the next editions.  And you know what, also?  In terms of just dropping some weird shit into your typical fantasy RP fare, you could do worse than lift a couple of monsters from the pages of GW.

Let's take the OBB.  30 years on, and the flying fungoid terror that flits mysteriously and ineffably over the surface of the wasted Earth, well, it haunts my nightmares.  I think I figured out that if the OBB blasted you, your chances of saving were slim to zilch... Let me find a thing, and we can confirm this.

You're screwed, mutie.  Totally screwed.

So, an intelligent, winged, hawk-like fungal creature that shoots a blast of radiation out of its eye and when you are inevitably dead, it lays spores inside you and it makes more.  And 3d6 damage PER CLAW!  AND 12 Hit Dice.  Although HD in GW are explicitly 6-sided, that could still mean 72 Hit Points.  30-40 average is nothing to scoff at anyways.  And (if you transition it rather seamlessly to DCC) it's immune to Color Spray, Magic Missile, and a bevy of other super duper Wizard spells that low-level characters rely upon.

Also, if you dig up the Radiation Matrix, you may notice that the Level 16 blast of radiation offers you (mostly) a 20% chance of mutation and an 80% chance of death - unless you have a 17 or 18 Constitution, in which case you just get a mutation a week later for sure.  I don't even know if I'd want to adjust the chances of this, it's so horrible and scary.  And fun.

The one time I saw an OBB in a module (Legion of Gold, incidentally, lost by me in my youth and then gifted to me again by Vincent) - it's in a dark and dead fallout bunker, if I remember correctly.  Imagine a dull flapping of the chitinous wings, followed by the single glowing eye of the OBB. It opens lazily at first, and then acquires a menacing aspect, finally blasting away when you don't get the hint to move on.  It could be totally silent and blinding, searing, ionizing radiation and then your party runs screaming and the survivors grow disadvantageous mutations a week later...

I'm tempted to make it psionic, also, but not in a dangerous way - maybe just mean and confusing alien telepathy to make your nose bleed as a warning sign before you run into the cold night air.