As a DM I love having allied NPCs of some stripe as part of the party. Having an in-character voice within the group of PCs can be so helpful.

It could be a more sapient familiar such as from Pact of the Chain, a hireling, guide, sidekick, or captured goblin the party befriends.

— Dan Dillon (@Dan_Dillon_1) December 11, 2020

Yeah we definitely picked it up from the henchmen concept from 2e.

We had a kobold rogue henchman that eventually was played as a PC. We also had a troll henchman for a time. It was cool as hell (In addition to the more normal humanoid friends that sometimes followed).

— Dan Dillon (@Dan_Dillon_1) December 11, 2020