Beastmasters can tame any of the monsters listed in the Bestiary, save for Hamsters at the moment. As such, it would be possible for a Beastmaster to get a T-Rex, Dragon or Behemoth, yes! For that dangerous monsters though, we request that the capture be properly logged and played out. It is not possible however to teach support skills (or offensive skills for that matter) to Monsters. These skills are only for Animals or Moogles.
To compensate for this, Monsters tamed by a Beastmaster can use the Blue Magic associated with them in the Blue Mage Job (in the case of a T-Rex, it would be Devour). They can also use any attacks commonly associated with them (such as flying for Dragons, all the things T-Rexes are known to do, etc).
Beastmasters can ride their pet into battle if it's big enough, but they won't get any advantage for using the Chocobo Knight Job while doing so. Chocobo Knight only works with Chocobo, so the only time when using Beastmaster and Chocobo Knight together provides the pet an advantage is if said pet happens to be a Chocobo.
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To compensate for this, Monsters tamed by a Beastmaster can use the Blue Magic associated with them in the Blue Mage Job (in the case of a T-Rex, it would be Devour). They can also use any attacks commonly associated with them (such as flying for Dragons, all the things T-Rexes are known to do, etc).
Beastmasters can ride their pet into battle if it's big enough, but they won't get any advantage for using the Chocobo Knight Job while doing so. Chocobo Knight only works with Chocobo, so the only time when using Beastmaster and Chocobo Knight together provides the pet an advantage is if said pet happens to be a Chocobo.