My understanding is that in sets from the last - 10 years? - there are Reverse Holos for the Commons, the Uncommons, the Rares, and the Holographic Rares. Holographic Rares are the "Super Rare" of the Pokémon TCG. There are times when a particular card is available as both a normal Rare and a Holo Rare (or all three), but those are special circumstances (namely, padding out a small set XP). I think the ID number is different between the Rare and Holo Rare versions in those cases.
Another one of those circumstances is what happened with Mewtwo (XY - Evolutions 51/108). The normal Rare and the Reverse Holo are from the actual set. If you have a holographic rare of it, it is probably the "Cracked Ice Holofoil" version from the Theme Deck (Mewtwo Mayhem). If there is a big "XY EVOLUTIONS" stamp on it, then it would be the Pre-Release promo; back then, Pre-Release Promos kept the normal set numbering. If you really like collecting that card, there's also the staff version; like the one I just described but with a gold "Staff" foil stamp on it, in addition to the set one.