You can find Mutant Gargoyle in the same place of Baby Gargoyle.

These are hidden in the trees among loads of Baby Gargoyles south of Behr Town. The Mutated ones drop down from the trees in a few locations and stick out since they don't fly like the babies and are a bit larger. They kind of look like giant angry hairless "flying squirrels" imo. My suggestion is run around spamming some AoE and mark the locations on your map/minimap of about 3 of them and run between those 3 points until you get the knowledge.
Credit: http://forum.blackdesertonline.com/index.php?/topic/39608-how-to-find-mutated-gargoyle/

There's a spot along the very eastern side of the area lpcrazy listed where 1 mutant spawns and nothing else will aggro you. I got knowledge by taping down my leftclick and taking a shower.

When i went to do this, the Mutant Gargoyles never were displayed. There would be a red dot on the radar, and when i aimed at that spot there would be a health bar but no visible monster. Attacking this health bar did result in getting the knowledge though. I just ran around killing gargoyles until i found a red dot without a visible mob.