I did a little rummaging on this here internet thinger, and from what I can tell, a horse dong is all meat, with no bone or anything, so, aside from shape and color, it's pretty much just like a guy's.
Sorry to say, I think you're wrong.
See:
http://en.wikipedia.org/wiki/Baculum
Dog's penis have a bone in it and they are not very "human-male-like".

Maybe it has to be a human one after all...