Here's a monster that popped into my mind yesterday ...
A creature hit by a reaper's blade and suffering 5 or more points of damage must pass a Fortitude saving throw (or save vs. petrification) or have one of their arms lopped off. The severed arm instantly comes to life and begins grabbing at its former owner or performing any other task the reaper sets it to. Arms have the same armor class as their former owner, and 1d4 hit points. If an arm is "killed" it is destroyed. Otherwise, the arm can be reattached (remember, there's magic involved) to its original owner once the reaper has been killed.
Some especially lucky (or talented) reapers ride strange mounts composed of severed arms. These mounts look something like centipedes.
Reaper, Small Humanoid, Average Intelligence: HD 3; AC 16; ATK 2 blades (1d6 + sever limb); Move 30; F13 R14 W14; AL Chaotic (CE); XP 300; Special - Sever and control limbs.
Manopede, Large Aberration, Non-Intelligent: HD 4; AC 14; ATK 2 slams (1d4+1 + grapple); Move 50; F13 R13 W11; AL Neutral (N); XP 400; Special - Grapple.
Severed Arm, Tiny or Small Aberration, Non-Intelligent: HD 1; AC varies; ATK 1 slam (1d3 + grapple); Move 10; F16 R16 W13; Special - Grapple.