I agree that it probably is a lower jaw from a horse! However, I don't think it is quite yet fossilized but it is very old, possibly several hundred years old or older and may have been washed out by flash flooding or a mud slide. Where was the lower jaw found? If in North America, I don't believe there were any large horse species (most fossils are from miniature horses) ever here until the Spanish came and it just might relate to some of their travels. Have the jaw bone checked out by the Archaelogical Department of a nearby University and they should be able to tell you how old it is and what species.
Frank