Grim Dawn: Definitive Edition

There are numerous different ways to unlock this achievement. Traditional pets obtained from mastery skills count, but also many skills that act like traps work just as well. Also, there are a few cosmetic pets that can be obtained from drops and as quest rewards. Even certain devotions, e.g. Widow's Arcane Bomb ability, function as pets.
I found that the easiest and fastest method is to start a new character and select Necromancer or Inquisitor as your mastery. Upon unlocking your second mastery, select the opposite. For this to work, you only need to be level 14, so this should go pretty quickly. Place points in the mastery bars of each, until you can unlock Undead Legion for Necromancer and Rune of Kalastor for Inquisitor. Next, put 1 point into the Necromancer's Raise Skeletons ability and 12 points into Undead Legion. This will give you 8 pets. For Inquisitor, place 1 point into Rune of Hagarrad and the remainder into Rune of Kalastor. The runes stay on the ground for a few seconds unless enemies detonate them, so clear an area, raise skeletons until you hit your max, and then keep dropping Rune of Kalastor and Rune of Hagarrad.