The stronger variants of monsters are called Champions. They are identified by a yellow name and usually there are no more than two of them per group of monsters.
Champions are stronger than regular monsters and they often wear armor. Usually they also have some special skills, like special attacks, movement/attack boosts, aura effects, and spells. They generally also have a better chance to drop items of higher quality.
Typical champion monsters are most of spellcasters, enchanted ranged fighters and dual-wielding melee attackers.