When attacking one, all dimprpodons in the area will attack you along with the one you attacked. When this happens, it is best to just take a weapon such as a pike and repeatedly stab the air. You will hit them as they fly by to attack you. Also, be careful even when you are on a mounted dino. They can attack the player while it is still on the dino, and can kill you. This happened to me as well; even though I was on a rex, a swarm of dimorphodons killed me while I was on the saddle. Luckily I was close to my base so I could just respawn and get my stuff back. However, you should still be careful about accidentally attacking on of these. Others can come out of seemingly nowhere.

More Dimorphodon Encountering Tips