How Rapid is Rapid?
Depending on speed and movement, an enemy can be more dangerous, harder to target, and more frightening. Use the different speeds stationary, slow, medium, fast, and quick for your enemies.
The difference between a hazard and an enemy is mobility and AI, but every rule has its exceptions. Just because an enemy is non-mobile doesn’t mean he can’t move. Movement = character and life. A humongous Cthulhu-esque creature may be to too big to walk or fly around, but players would still consider him an enemy. Even an “inanimate object” like a turret can be smart enough to make life difficult for players. Design ways to keep players engaged as they attack non-mobile enemies, whether it’s a timing puzzle that stands between the enemy and the players, or even a puzzle that’s part of the enemy itself.
The speed, size, and strength7 of an enemy are inversely proportional: small enemies are fast but not strong, big enemies are strong...