Hi, Veronica and Bruce–
Thanks for the e-mail! The tooth looks to be a lower right cheek tooth. It’s definitely not Equus, and to my eye doesn’t look right for Dinohippus, Pliohippus, or even Astrohippus. That stylid on the anterior end looks interesting. If you take a look at the appended pdf, specifically Figure 4, you’ll see lower cheek teeth of Calippus that appear to be similar, and are approximately the same size. That’s not to say that you should commit to your tooth being Calippus – I’m no expert on horses of this geologic age, and the fossils described in the appended paper are Miocene rather than Pliocene. But you can be confident that your tooth is very definitely a fossil, and likely worth accessioning into your collections. I hope this helps!