I've seen various artists, when rendering this beasty, perhaps take Lovecraft too literally when he says "horse-headed"; meaning they literally either render a horse-head or horse-skull, when Lovecraft uses the term in the same sentence where he compares its size to that of elephants:
"for they were larger than elephants and had heads like a horse's."
Perhaps it's something that's arguable, but I can't really see Lovecraft making it a bird and giving a bloody horse's head. With this in mind I've made the body roughly the size of an elephant's and made the skull proper (the lower jaw is not considered a part of the skull) about the size of a horse's--though this skull-to-jaw ratio is just something that I've been playing with and may change in the future if I come back to update it...
I finished this too fast so I could post it in time for reddit's /r/lovecraft forum, so I'll probably go back and spruce it up later.