I remember this movie from the late 1980's / early 1990's. It was a VHS rental, and may have been a straight-to-video release.
The protagonist is a human aspiring to become the champion of an arena that has a "handicap field" that equalizes the physical strength of the combatants. Most of the combatants are aliens; no human has been champion in many years.
The reigning champion of the arena is a horned humanoid alien named "Horn". Horn is of low intelligence - the only word he can say is his own name - and there is a conspiracy to ensure that he remains champion.
The protagonist's first fight is against a huge alien with many limbs, named "The Slug". The conspirators are not yet concerned that the protagonist might become champion, and the handicap field functions normally, allowing the protagonist to fight at 100% strength, but restricting his much larger opponent to 60% strength.
When the protagonist challenges Horn, we learn that Horn has no respect for the laws and customs of the arena; Horn enters the arena first - the custom is for the challenger to enter first - and immediately headbutts a non-combatant - either an arena official or one of his own coaches.
The conspirators are now concerned that the protagonist could become champion, and they hack into the computer controlling the handicap field to bias the fight unfairly in Horn's favor. The protagonist's allies discover this and disable the handicap field completely.