If you’re in a hunt of Megaloceros and you wanted to tame it, always bring a bow and tranq arrows since it is the most effective and cheap way to knockdown a big daddy reindeer like that. Another tip is hit the head of the Megaloceros so that the torpor effect will trigger quicker than the average and it will knock him down quickly so he doesn’t have time to run away from you.
To tame a Megaloceros, I would try to tame an Argentavis and carry it to somewhere warm. Assuming you have plenty of Mejoberries from any berry collector, or even by hand, or you could have Dimorphodon kibble you can begin taming. I would suggest using a bola, because these guys like to run. With a few tranquil arrows to their head they would be out quick. These guys don't usually need much narcotics. The megaloceros is amazing at collecting thatch, and they have a bleed effect, which is great for pvp and killing dinos.