Molten Crucible, Aberrus, the Shadowed Crucible (Raid)
Retains its original name after taming. Tameable in Normal mode, so you don't need to enter a higher difficulty if you don't want to. To reach Magmorax: after killing the first boss (Kazzara), turn left at the end of the path and defeat the next three encounters: Amalgamation Chamber, Forgotten Experiments, then Zskarn. After Zskarn you can drop down through a slot in the balcony floor to the magma below. Kill all the Voracious Magma Worms surrounding the central platform to activate Magmorax. Note that this boss will respawn very soon after taming, so you can tame another, or kill it! It's possible that using the raid skip will prevent you reaching this boss.
Level scaling: Most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.