Alola added location-based evolutions unique to the island setting. Charjabug evolves at Vast Poni Canyon, and Crabrawler evolves at Mount Lanakila. Alolan forms change classic chains: Alolan Exeggutor gains Dragon typing, and Alolan Vulpix swaps Fire for Ice. No Mega Stones are available until the postgame.
Alolan Vulpix becomes Ice-type Ninetales instead of Fire. Alolan Exeggutor swaps to Grass/Dragon. Alolan Geodude's line becomes Rock/Electric. Alolan Grimer's line becomes Poison/Dark. These aren't just cosmetic; they're completely different types and sometimes learn different moves. If you catch them in Alola, you get the Alolan form automatically.
Charjabug evolves at the Vast Poni Canyon magnetic field area. Crabrawler evolves at Mount Lanakila. Eevee becomes Leafeon near the mossy rock in Lush Jungle and Glaceon near the icy rock on Mount Lanakila. Each location is a one-time visit during the story, so try to have the Pokemon ready when you reach those areas.
Rockruff evolves at level 25, but the form depends on your game and the in-game time. Sun gives you Midday Form (evolve during the day). Moon gives you Midnight Form (evolve at night). The Dusk Form version requires a special Rockruff with Own Tempo, which was an event distribution.
Not until postgame. Mega Stones aren't available during the main story. After becoming Champion, you can collect them from various NPCs and locations throughout Alola. Z-Moves are the generation's main battle gimmick instead, so Mega Evolution takes a back seat until endgame.
Max out its friendship and level it up. You get Type: Null as a gift from Gladion after becoming Champion. Since it starts with low friendship, use the Soothe Bell, vitamins, and walk around with it. It evolves into Silvally, which changes type based on its held memory disc.