I think you guys are being a bit unfair. I'm sure this would have a 4.25 rating if one of the older hands had put it forth.
As for myself, I think that it still needs some work in terms of additional plot hooks. We could all probably come up with a few good uses for this toxin (Wulfhere's already started!), so why not put a few forth. Flesh it out a bit. What exactly is a Howler Demon? Why should we be afraid of it? Do Demons reproduce sexually, and even have pheremones?
I could see a large problem with the respiratory and circulatory systems- the lungs and hearts of both animals would need to be enlarged to cope with a sudden increase in mass. Would the human head get too hot, as it's less efficient at shunting excess heat than its equine counterpart?
Otherwise, a very nice way of making a legendary creature more palatable and realistic. Good work. Go to Comment
Teenagers who run away from home to live in the wilderness may become feral. As they become more and more feral, they gain some supernatural powers (a la Peter Pan - ability to fly, things they imagine come true - possibly quite powerful), but at the expense of their humanity and ability to speak, comprehend complex subjects, etc. Maybe they have a leader: the most feral of them all.