Overlord II

This is story related because at some point during the story you are required to use all three mounts.
The browns have wolves and you use these while in nordberg and the surrounding areas.
The reds have salamanders and you use these during the level where you get dark/ghost fay. after you fight the salamander king.
th greens have spiders that you get in the everlight jungle after defeating the spider queen.

This achievement is unmissable

SPOILER

Throughout the game you will find mounts that your minions will be able to ride, in total there are 3 mounts for 3 types of minions, Browns, Reds and Greens.
The first are the wolves, you must ride them right at the beginning of the game in Hunting Grounds or Nordberg Town, the second is the Spiders that you must get after killing the Queen of Spiders between Everlight Temple Outskirts and Everlight Temple, lastly there are salamanders, you should be able after to defeat and destroy the head of a giant Salamander in Westland Sanctuary, the achievement will be unlocked as soon as you pass a ramp with the fire salamanders.