Hireling

Hirelings was a term used to refer to non-Follower NPCs during Diablo 3's development. These occasional companions were basically wandering NPCs who were occasionally found in dungeons and would tag along behind your hero for some time; until they died or completed a mission.

These types of NPCs are still found in the game, largely in the form of frightened or lost guards in Act Three, but also during some quests in other acts. Leah and Tyrael could be described as a Hierling of a sort, as they accompany the hero on several quests in Acts 1-4, but do not receive equipment, level up, take damage, etc.

These NPCs are of no real importance in the larger game and the significance of hirelings grew less during development until they were merely in-game color by the time of launch in 2012.

Hirelings During Development

Leonard Boyarsky shared some of their plans for Followers and hirelings at Blizzcon 2008, though these plans changed greatly before the game's launch four years later.

We have two different types of mercenaries now. As you saw in the demo, if you got the quest where you could rescue the adventurers or their leader, those guys are a low level and are just along for the quest, or are cannon fodder. You can't really control them or have anything to do with them at all.
When you have what we are calling followers, they are the guys you can equip, give them different weapons, you can give them different armor. They will probably have some quests that involve them. Much more than in Diablo II, you could equip them but they were more like a game mechanic in a body of an NPC. Where this time, were making them much more individuals with their own back story and their own reason for being in the world.

NPC Hirelings

Hirelings were seen on many of the quests during the Blizzcon demos in 2008, 2009, and 2010. Several archers were the best known, appearing midway through the introductory Diablo 3 demo, released at Gamescom 2008 when the title was announced.

NPCs assist the Barbarian to rescue Cain.