Easy tame, just use bolas, trap/protect it with spike walls, feed berries, and use narcotics/narco berries to keep torpor up. I play on a small tribes server and tamed a max level in less than an hour with 3x.
Deers are easy to tame because all you need is to find a good level deer and bola it and punch it out or tranq it or club it. And feed it meoberries. Also narcotics or narcoberries to help to stop it from waking up.
My favorite cave mount. I have been breeding them for months, my 776 melee outputs as much damage as a wyvern bite at 500 melee, plus bleed. Put an MC or better saddle on one and it will wreck any cave. They absorb otherwise rider inflicted damage, they can jump to clear gaps, they're fast, they output massive damage, and can hit more than one target at once with their attack. What's not to love? (Low health and weight which also makes them painstaking in baby stage)
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.
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.