A Pat, short for Patrol, is a patrolling mob or mobs. Pat refers to the whole patrol, so if you have a linked patrol of three mobs, it is still just refered to as a pat. Patrols are also sometimes referred to as walkers.
Unlike other mobs that may wander around but stay in a small area, pats walk back and forth along a set path that may circle or pass through an area. A pat will cover more ground than the typical mob area but will stay on their set path.
In many instances, patrols are known for respawning faster than the other mobs in the zone, or in the case of Deadmines, spawning as the players progress through the dungeon.
While a pat is generally not linked to the more stationary mobs, pulling those mobs with a pat too close (or vice versa) will result in adds.