How is stephano's mistake, on seeing the other two together, partly understandable?

First of all Stephano has been drinking so his judgment is a bit off. He hears Caliban cry out to Trinculo, “Do not torment me! O!" Caliban's voice probably didn't sound very human! Stephano then sees four legs under a cloak and assumes it is some kind of monster. It is a bit of a stretch to think it's a monster but strange things were happening on that island.