Achelous, Asterion, Bullman and Minotaur are four different types of creatures found in Ancient Greece. They are similar but not identical, and differ slightly in their appearance (body color, bracers, shin guards, ...). Bullman Minotaur Asterion Achelous Bullman and Minotaur can be found between Sparta and Olympia but they are relatively rare. On the contrary, Asterion and Achelous are common. Ephesus (for Asterion) and Olympia (for low-maturity Achelous) are two popular places.
True, Asterion and Achelous are way easier to find than Bullmans and Minotaurs, but all the mino spawns are way thinner than the Cyclops ones.