Creature is a term for all entities a hero can encounter or hire in Heroes of Might and Magic III. The word is interchangeably used with unit, troop and troop stack. Typically word troop and troop stack are used when talking about hero's army. The original Restoration of Erathia manual states, that armies are made up of creatures arranged in groups called troops.
In the adventure map creatures appear in wandering creature stacks. Without Visions spell or Rogues in the army, a player can not see exactly how many creatures there are, but they can see a rough estimation as seen in the table below.
On the battlefield, Creatures appear in stacks. Stacks take their turns according to initiative and can move and/or attack on their turn. When a stack takes damage, the 'top' unit of the stack loses health, and when it reaches 0 health it reduces the size of the stack by one and reveals the next-top creature, which starts with full health (unless the damage taken exceeds the current health of the previous top creature, in which case the new top creatures loses health accordingly). A stack deals damage according to the number of creatures in it. When the last creature in a stack dies, it leaves a corpse on the battlefield which can be the target of spells/abilities like resurrection. All of the creatures that were in a stack at the start of the battle are in the corpse (rather than, say, the stack leaving small corpses behind every time any of the creatures in it die).