A wand of slow monster is a wand that will make monsters slow down (take more turns to move or attack).
This is an excerpt from Spl1-343.txt:
Zap monster Edit
The monster becomes slow, or normal speed if fast. If you are engulfed by a vortex or air elemental, you are expelled. Some monsters may resist.
"You disrupt <monster>! A huge hole opens up..." (expelled) "<Monster> seems to be moving slower." (slowed, not swallowed)
Zap self Edit
If you currently have the speed intrinsic, it is lost.
"You slow down" (not wearing speed boots) "Your quickness feels less natural." (wearing speed boots)
It can be identified by engraving: it will make the bugs on the ground slow down.
Wands comprise 4% of all randomly-generated items in the main dungeon, 6% in containers, 5% on the Rogue level, and 8% in Gehennom. There is a 5% chance that a randomly-generated wand will be a wand of slow monster.