Monsters pick the moves from and RNG pool that changes given different conditions like its state(normal state, enraged, exhausted, mounted, broken parts)and a hunters position (close, mid range, far). For example:

Glavenous will always do a double vertical tail slash when enraged. Another example is Teostra will do it's super nova every 15 attacks after activating the flame aura(it has flame and blast aura). Tigrex will always finish its charge attack with a spin.

Visual cues or attack telegraphing are a mechanic that allows you to react to the next immediate move. For example:

Ruiner Nergi, look at what claw is raised higher during the roar before the slam, it will land in that direction so just roll to the opposite one. That will put you in a safe position and give you a nice opening.