you know its there. Find it and shoot it before it sees you.
This has nothing to do with single player. Those are the highest level normal enemy in the game (things in the plains biome, that is) and they will flatten you same as a wraith will melt someone just starting out in the meadows or a grey dwarf brute will smash a guy wearing skins and wielding a stone axe. These enemy are all fine single player if you are at the appropriate gear & food level to be fighting them.