If you get too close to a wild one that used it's special ability (a charge that will set it on fire and aggravate nearby dinos) it'll display a message saying "getting hot! Stay away!!!!" and that will set you on fire. Don't approach unless you know you're prepared.
The magmasaur has a very very bad turning radius when trying to kill one try to continuously try to be behind it. It’s flaming aura when using its spit will hurt you, the magmasaurs body will become in fire whilst doing this attack that will be your warning. Magmasaurs also come in packs of 1-3 so if you hit one another one will most likely be there