I have personally had trouble finding these guys on single player, but I found a tip that will help you guys! All you have to do is eat rare flowers and everything will aggro on you including these guys. They spawn in the redwood cave, carnivore Island cave the artifact of the hunter cave and the lava cave.
A quick tip for ragnarok players, there's enormous amount of megalania in Highland near dragonmalte tranch - beach entrance. Once you on there don't you dare to run especially if you have a high speed stats, since this thing's height is almost the same as moschops so you wouldn't reflect to avoid one of these, don't worry, unless you never get too close to them you will never gets agroed by them, the agro meter is really low even lower than a theri.
You can bola them, but if it chasing you you'll need 2-3 bola in order to get them stay in place.