IVAN take on a Medusa, you say? Hmm...
Apsara are Hindu cloud spirits. According to one legend a set of statues in a certain throne room somewhere are actually Apsaras who were cursed. What if there were Apsara in IVAN who would disguise themselves as statues until the PC was close? (This would require actual statues to be a thing that could be found occasionally...) On a similar, more European vein, there could be gargoyles with the same ability...
According to legend, the "Cap-Chat" was a cat who ate the children of the "cat fairy" and was turned to stone as a result. How about a Cap-Chat monster, who gradually turns your limbs to stone with claw attacks?
Or, we could keep the traditional medusa, except change the ability... What if there was a twisted, distant relative of the frogs - neither lawful nor chaotic, but simply evil... the Toadon? (Or some other play on Toad and Gorgon/medusa/etc.) They could have the power to transform your limbs and gear into different materials with a glance (simulated in the game similarly to a wand beam). I'm thinking it would transform you/your gear into softer materials. Specifically, I'm thinking mud. The transformations would be temporary, but duration would stack with subsequent attacks - and of course if your limb was severed, the transformation would be permanent (probably your arm just explodes in a shower of mud as soon as it's hit with much force). In order to not be too devastating, I think the attack should apply in stages as the duration extends - if the duration is almost over (i.e. short) your limb might not really be affected; if it's longer you might lose some strength/agility/HP; if it's really long your limb material actually changes until the duration becomes shorter.
"Put more stuff in the... thing where... more stuff goes in."