Illusions:

Illusions are temporary copies of a hero in Dota 2 that look and often act like the original, but are weaker and cannot use most items or abilities. They are created by spells, items, or hero abilities, and they are used to scout, push lanes, farm, and confuse enemies during fights.

Many illusions deal and take different amounts of damage from the real hero, so understanding which unit is the true hero matters in combat and when clearing waves.