How to tame Meg:

First get about 12 tranq arrows and place them in a cross bow

Mount icthy and find Meg

Get megs attention

When the Meg starts to chase you lure it into shallow water and start shooting it with your crossbow (you may have to dismount your icthy)

When the Meg is unconscious go as close to the Meg as possible and dismount your icthy and give the Meg about 30 meat

Wait about 5 mins and enjoy your new tame!