Children of Morta

A ferocious enemy is any enemy with the yellow or red symbol above them - not just the red ones that the achievement image shows.
They spawn randomly, like everything else in the game. You should pick this up naturally during your progress through the storyline, but if you need to grind them out then run through the first level with an OP character to make quick work of them.
There are usually a few of them during the strange beast encounters (that reward you with a mysterious egg). Once you have nabbed the egg you can backtrack through the level and fight the enemies for their loot and progress towards this achievement.