The Wrist Gun is featured in Predator 2 and The Predator which allows the Predator to fire a blast from his wrist gauntlet.
Predator 2 (1990)
City Hunter uses this weapon in the meat locker in Predator 2 and fires off a powerful rocket from his wrist which hits Harrigan. The Predator used this after its shoulder cannon was destroyed which suggests the wrist gun is a sort of backup weapon to the shoulder cannon.
The Predator (2018)
The Upgrade Predator has a very similar weapon in the absence of the usual shoulder cannon. A miniature cannon can emerge from his wrist gauntlet which can fire a plasma blast.