Rider in Mirkwood

I was rereading The Hoobit the other day when i came across something that bothered me. In the chapter "Flies and Spiders" while crossing the river, there is some rider or animal that comes down the trail leaps over the river knocking Bombur in the river causing him to go into a deep sleep. What was that thing it always bothered me. After reading the Simerillion i thought maybe is was that Valar Olme, the hunter one, but now i am not to sure.

please forgive me if the name of the Valar is wrong, it has been so long since i read the Simerillion.