When seen from a distance this goat looks unusual, but not impossible. It is dark. Black, in fact. It has 4 horns, which normal goats sometimes have. There is something disturbing about its back.
When you can see it properly, you see the dozens and dozens of moving eyes on its back and shoulders. The eyes are of many sizes. They gaze in many different directions. You dare not look closely enough to see if they are of many colours. Then you see the many mouths opening and closing.
Then you see the intermittent dripping. The dripping of eyes. The eyes slide down its sable hide to the ground and grow. They grow and transform.
1d6 eyes and mouths slide from the goat's back and shoulders each round, each growing to a full Spawn of Shub-Niggurath in one round. The goat itself is an avatar of Shub-Niggurath, and as such is virtually indestructible. As an instantiation of the Old Ones, it is also all but oblivious to mundane creatures, such as adventurers. Due to its constant propagation, it is horribly easy to follow its trail, though perhaps unwise to do so.