This is something you see in heroes as well as villains. As a way of justifying how great a character is at fighting (including their Charles Atlas Superpowers), part of their backstory involves them fighting in some form of gladiatorial combat. Bonus points if they earned their freedom that way.
- Conan the Barbarian, in the movies anyway, after his stint at the Wheel of Pain, was taken (presumably by the same people who put him there) and sent to the arena, from which he emerges in full "lahmentashuns of da wimin" mode.
- Several characters in Mahou Sensei Negima!, most notably Jack Rakan:
"He made his debut in the arena, a young slave gladiator in the Empire. And over the first few years, he almost died any number of times. It was only after he climbed to the peak of the gladiatorial world and became a freed slave that people began to call him "invincible"...his strength was forged and tempered over forty long years of real battle experience and training."
- Megatron in Transformers Prime gained popularity as a gladiator that he later leveraged into political power, then rebellion, then planet-making-bad...
- Pumyra from the 2011 version of Thundercats is found by the main cast having been sold into an arena run by dogs after Thundera was destroyed.
- Before hitting the big-leagues, Dr. Impossible of Soon I Will Be Invincible spent several years in the Underground Supervillain Fighting circuit in Thailand, using stage names like "Smartacus".
- Sagat, in the movie version of Street Fighter, was an underground pit fighter before he graduated to running the fights himself.
- Thrall, Warchief of the Horde in the Warcraft universe, was raised for gladiator combat before he became a shaman.
- The Gladiator character in Sacred is Exactly What It Says on the Tin.
- Weaponlord has this as almost a requirement for the roster, as this is set in a time-period where barbarians were commonplace. The Demon Lord Zarak runs his own arena circuit, with Jen-Tai as his arena queen, who controls the fights and the arenas.
- Strong Belwas from A Song of Ice and Fire is one of the toughest fighters in Daenerys' entourage due to having spent most of his life as a gladiator slave.
- Dungeons & Dragons 4th Edition has a number of character options built on this trope.
- The character backgrounds grant the bonuses or training in the skills they'd have used as a gladiator, mainly athletics and endurance. The Follower of Kord background explains that the character learned to worship Kord, the god of strength, during their time as a gladiator. The Former Gladiator background grants the character an additional death save due to their hardened past.
- The Dark Sun campaign setting features a theme that grants a character additional powers that they picked up during their gladiator days, outmaneuvering and exploiting their moves.
- Spartacus: Blood and Sand: Oenomaus was a champion gladiator before becoming moving on to overseeing the rest of the Gladiators. This made him the biggest threat to Spartacus's rebellion early on.
Hello, Unknown Troper. You'll need to get known to lend a hand here.