There is no reason, biological or otherwise, that these creatures shouldn't be able to exist. Unlike ancient tales of fantastic creatures, these specimens apparently conform to known parameters. (Pardon my two-dollar words. I just came off of a cryptozoology site.) What is questionable about these creatures are the sightings themselves. Some seem to defy explanation, while other look like a guy in a gorilla suit. I believe that these are terrestrial creatures that have simply eluded us. There's a species of either bull or deer that was fairly recently discovered in Cambodia, the existence of which was previously unknown. Unfortunately, since they apparently won't be taken alive, we can't know for sure until someone shoots one of these creatures and brings it in for dissection.
And it would be just our luck that it'd be the last male or female of the species.