Generation 1 has a unique aspect in that it is the only generation that had introduced three fossil Pokemon instead of the normal two that Generation 3, 4, 5, and 6 introduced. In addition, that third fossil Pokemon is the only one that does not evolve but has a mega. The Pokemon I am talking about is Aerodactyl. In Red, Blue, Yellow, Fire Red, and Leaf Green, this fossil was available to you always, unlike only being able to choose the Helix Fossil or Dome Fossil. However, it wasn’t in the main story line, so if you didn’t know about it, you might have missed it. What you need to do to get it is go back to Pewter City once you get Cut. In the northeast corner of town, there is a cuttable tree. Cut it, then go inside the building. There will be a guy next to a display stand that says he believes his Old Amber is a fossil, but nobody else believes him. He asks you to go to Cinnabar Island and revive it (which is also where you can revive your other fossil). Doing so will get you the Rock/Flying type Aerodactyl. This was a unique typing until Generation 5 introduced the Archen line. Still, its a good typing. It removes Rock’s weakness to Ground, while also making Grass and Fighting moves do regular damage. Of course, this comes at the expense of it now being weak to Electric, Ice, and Rock, but Ice types shouldn’t be too much of a problem. Anyways, Aerodactyl is available in Sun and Moon, and you don’t need to buy any fossil to get it. Someone in the Seafolk Village gives it to you for free at level 40. But should you bother getting it?
By the time you get to Seafolk Village, most of the game is done. As for the Elite Four, it would not do well against Olivia’s Rock types. For Hala, there are better options. For Kahili, it would be a good pick if you didn’t have any Pokemon good against Flying types. Against Acerola, it would be good for some of her Pokemon, like Drifblim, Dhelmise, and maybe Froslass, but not so good for others, like Palossand. All in all, you should get one for the Pokedex, but putting one on your team would seem forced.