Need a monster, or maybe a magic item

[NeoTiamat]

I believe I mentioned before the Catspaws from VRA. Give them to just about any character and the PCs will think they have a weretiger on their hands.

[Gonzoron of the FoS]

This is Darkon, so a bit of weirdness is allowable without getting burnt at the stake. I didn't want to get too deep into the specifics so as not to bore anyone, but here they are:

some time ago, Dr. VR assisted the town guard in capturing this monster. Since it was close enough to "normal" not to be slain outright, it's been in prison. Now the same killings are starting again, but the monster is still securely in jail. What gives? member of town watch writes to Dr. VR to ask him to come back. VR is busy and sends PC's instead. What's actually happening is that a (Kargat) vampire is dominating the jailkeeper and the monster, letting the monster out to do some dirty work, and then putting the monster back, with none the wiser.

The Scooby Doo approach would be good in general, but this is meant to be a small piece of a larger puzzle. They aren't going to need to figure out what this guy is. (Dr. VR knows and will tell them.) What they need to figure is that he's getting out and how he's getting out. (i.e., it's not a copycat, or another of the same monster.)

[Jester of the FoS]

A pristine walking dead (think pet sematary) might work. One similar to a ghoul without the purple skin and stench. It's believed to be human (it looks human) but eats its victims and needs raw meat to heal itself.
However, I doubt these could be controlled/dominated either...

Something animalistic might be the best. A human barbarian cursed after several failed Dark Power checks or a serial killer/madman. Maybe a broken one that has a very human face and dresses in robes.

[Rotipher of the FoS]

Even in Darkon, people tend to execute monsters rather than jail them. A captured creature that's obviously non-humanoid (like an aranea), and that's believed to have been killing people, would certainly be destroyed ASAP ... unless, of course, the Kargat make it 'disappear' from prison so it can be interrogated and/or conscripted into their own ranks.

Instead of a monster, how about using an outlander of a humanoid race or subrace nobody's ever heard of? A drow or duergar might have racial special abilities weird enough (and be spooky enough) to merit a consult from Van Richten, yet isn't overtly "monstrous" enough to be slain out-of-hand. Other oddball races from non-Ravenloft supplements might also be suitable, from goliaths to phanatons to Irda -- heck, even a plain old orc could suffice -- if initial reports from witnesses are distorted by fear and portray the prisoner as more "supernatural" than it actually turns out to be.

Either way, I'd recommend that the captive's initial guilt be left open to question, so it's plausible that it'd be kept alive to stand trial in the first place. Darkon isn't exactly a shining bastion of civil liberties, after all: the locals might not burn a drow at the stake just for being a drow, but they'd certainly behead one without hesitation for being a murderer.

[NeoTiamat]

That's a very interesting idea just there. An outlander of some sort. You could do something similar with an unusual PrC. Theres something like 800 PrCs around, so I'm sure you can find something distinctive.

[Gonzoron of the FoS]

Even in Darkon, people tend to execute monsters rather than jail them. A captured creature that's obviously non-humanoid (like an aranea), and that's believed to have been killing people, would certainly be destroyed ASAP

Exactly. Which is why it needs to pass for human(ish). Van Richten knows it's a monster, but couldn't prove it sufficiently to the authorities for it to be executed. An aranea can apparently take a fully humanoid form if it wants. "Well, it's plain that this odd fellow did it alright, but how he made those webs, I guess we'll never know..."

Instead of a monster, how about using an outlander of a humanoid race or subrace nobody's ever heard of?

hmm... perhaps...

Either way, I'd recommend that the captive's initial guilt be left open to question, so it's plausible that it'd be kept alive to stand trial in the first place. Darkon isn't exactly a shining bastion of civil liberties, after all: the locals might not burn a drow at the stake just for being a drow, but they'd certainly behead one without hesitation for being a murderer.

Exactly, that's what I'm going for. Van Richten knows it's a monster, and possibly one or two others in the guard who saw it in action. But the higher-ups are skeptical enough to keep it alive and jailed. (As mentioned before, Tooms from X-files is a perfect example, if only his power didn't involve squeezing through small opennings.)

[Brandi]

Tooms would slip right through the bars, wouldn't he? If memory serves, he once escaped through a food slot in a door.

Ayuh. Mind you, I figured he avoided your "don't turn into an ooze" caveat; IIRC he's just inhumanly flexible.