It is conventional[1] to distinguish two categories of bestiarii: the first were those condemned to death via the beasts (see damnatio ad bestias) and the second were those who faced them voluntarily, for pay or glory (see venatio).
[1] Seneca relates that a German prisoner, rather than participate, killed himself by forcing a sponge used in the lavatory down his throat.
[6] Bestiarii, as reported by Seneca,[1] consisted of young men who, to become expert in managing their arms, fought sometimes against beasts, and sometimes against one another; and of bravos who, to show their courage and dexterity, exposed themselves to this dangerous combat.
Augustus encouraged this practice in young men of the first rank; Nero exposed himself to it; and it was for killing beasts in the amphitheatre that Commodus acquired the title of the Roman Hercules.
It appears that there were schools in Rome, in which people were trained to fight wild beasts (scholae bestiarum, or bestiariorum).