[RPG] Is there such a thing as a Magically Animated Coin Monster

So far as I know, animated objects are the result of an animated object spell, nothing else. Hiring an NPC to cast animate objects and then permanency would cost 16,540 gp minimum (caster level 14th for the casting of permanency on animate object; at that CL you could get a Huge one).

Crafting a Construct by the Craft Construct rules would result in a creature not vulnerable, as a permanent animate object is, to dispel magic, which is a rather huge advantage. On the other hand, it takes longer to set up (the whole crafting time), not to mention the Craft Construct feat itself, so it perhaps makes more sense to compare it to other Constructs that you can craft. It’s better than a Flesh Golem (20,000 gp) most likely, but not as good as a Clay Golem (40,000 gp). I’d probably put it around 25,000 gp then, base cost.

I believe this is the Lock Lurker from several AD&D 2e publications. It was first featured in Forgotten Realms creator Ed Greenwood's regular column, "The Dragon's Bestiary," in Dragon magazine #139 (Nov 1988, pp. 70-71). Next, Greenwood put it in the Forgotten Realms adventure Haunted Halls of Eveningstar (1992), and then the Forgotten Realms Campaign Setting (1993). It was also in Monstrous Compendium Annual Volume I (1994) and lastly in the Priest's Spell Compendium Volume III (2000).¹ (Special thanks to @SevenSidedDie for additional research!)

The fullest descriptions, along with pictures, are found in Haunted Halls of Eveningstar, Forgotten Realms Campaign Setting and Monstrous Compendium Annual Volume I.

Reproduction of stats for AD&D 2e is likely copyright infringement, so I won't be posting anything too specific, but ...

It looks like a coin—a cold, hard, coppery or bronze disk (25% are silver or golden). It has two rows of tiny, retractable legs on its underside, surrounding a razor-sharp iris of teeth, and a lightning-fast stinger that can be up to a foot long, but this stinger is usually on the Ethereal Plane, invisible to observers on the Prime Material.

A human handling a lurker often thinks he has picked up a smooth, heavy coin until its sting advises him otherwise. ...

The stinger can attack creatures in the Ethereal Plane and materializes on the Prime Material Plane only when the lurker launches an attack ...

The stinger "is powerful enough to pierce any armor" and contains 40+ uses of venom that can stun, slow and potentially paralyze victims.

A google search should give you any other information you may want.

1. The lock lurker is described in the Priest's Spell Compendium Volume III as part of the summon lock lurker spell (p. 617), exclusive to priests of Waukeen, which summons 1d3 lock lurkers for 8 hours that are "typically set to guard treasure, small areas such as the interior of chests, bags or pouches, or the person of the caster."