Rathalos was added in patch 4.36 of the Stormblood expansion.
Rathalos becomes available at level 70 as part of the “The New King on the Block” quest, which is given by Hearty Hunter – Kugane (X:9, Y:8).
The trial has a minimum ilevel of 320 to enter and rewards you with:
Allagan Tomestone of Poetics: 10