I've been looking for an hour or so and I've found 2 magmasaurs. I haven't found the egg cave yet but I'm going to keep looking. The first one I found was on the N side of the W volcano about half way up. It was just outside a cave full of chitin (mantis, scorp, centipedes). The second one was on the S side at the oil pools
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.