How to ride a flying dinosaur in Horizon Forbidden West

In short, this can only be done as you move along the storyline. I don't want to spoil it, so I'll give you a simple answer: be patient and go through the storyline, and only closer to its completion you will be able to ride a flying dinosaur.

And with such a dinosaur, it is easier to connect all the Long Necks. That is, you won’t have to suffer with them as in the first part! Moreover, there are Longnecks, which simply cannot be climbed without a flying dinosaur. After a certain story point, you will be able to capture these dinosaurs and summon them like a regular lenter. You don't need to capture any Cauldrons!