Wanna find one? Simply mind your own business flying through the redwoods and they'll find you! Just make sure you can take some fall damage and that your tame is set to passive so it doesnt kill the thyla. (I know this seems like a joke but this is legit the only way I know how to find them.)
If you wanna find them easily just tame a parasaur and use an arge to carry it into the red woods. Have the parasaur on turret mode so it can tell you where the thyla is. It may take a while but I see this as the safest way to find them. You just have to move slowly through.
The redwoods are now basically a no fly zone, I lost a quetz, and a pter to these b******s. They jump out of the tree, rip you off your mount, and then pin you and your mastercraft armor to the ground and murder you.