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.
When in areas that have many dimorphadon remember to be careful and not to hit one by accident when fighting other creatures as you can easily be overwhelmed.your mount will not be able to save you as they deal damage directly to the rider.
I went to raid a base with 4 buddies, a rex, 3 carnos, and a bronto with a platform saddle and cannons. All we heard was a whistle and i swear about 50 of these little devil's swarmed the whole group, killing everyone in the group and one carno, the turrets shredded everything else, but the rex must have ran off and we never got a death message so its still out there in the jungle.