Can be found in Carno island cave, south west lava cave, south east cave near Stonehenge, Central cave near volcano, and possibly others. They are very rare spawns but if checked regularly you can encounter them if looking to tame.
Fun fact: megalosaurus (name translates to “big lizard”) was the first true dinosaur found. Discovered by Richard Owen, he thought it was quadrupedal (walked on all fours) and resembled a crocodile-lion cross. Later he would correct his data and soon classify this group of extinct line of ancient lizards as “Dinosaurs”.
Whatever you do, do not get close to it with pretty much any medium dinos or fliers because these thing can pick up fricking argies and monch them to death so keep a good distance between them, you and your tames. Trust me on this i lost a lot of things to these monsters.