Homebrew Wandering Monsters

When you roll a 12 on the Wandering Monsters table, or if you just feel like it, you can use one of these creatures.

Dopplegangers
By Premier at the Dragonsfoot forums.

Like all wondering monsters, Doppelgangers have a 1 in 4 chance to move every round. If it moves into the ZOC of a Delver or Breeder monster group, or if the ZOC of such a group expands to the Doppelganger, a d6 is to be rolled: on 1-4 the Doppelganger infiltrates the monster group, on a 5 it's chased away (moves 1 finger out of the ZOC), on a 6 it's attacked according to the wandering monster combat rules.

If it manages to infiltrate a group, the Doppelganger counts as one unit of population for purposes of breeding and population support, but it's NOT counted when determining combat strength, and it cannot be removed as combat loss. Every subsequent round, the Doppelganger has a 4 in 6 chance of trying to assassinate one unit of population. For the attempt, a d6 roll is made for the Doppleganger and another for the target, with the following results:
 * Win by more than 1: one is removed, and one loot marker belonging to the monster group is removed and set aside off-table, in the possession of the Doppelganger.
 * Win by 1: one is removed.
 * Tie: Doppelganger flees from the monster groups's ZOC, taking with it 1 from its plunder. The rest of the plunder is recovered by the group and placed back on the map.
 * Lose by 1: Doppelganger flees from the group's ZOC. All of its plunder is recovered by the group and placed back on the map.
 * Lose by more than 1: Doppelganger is killed, its plunder recovered.