Pokemon not normally found in Alola can be found through Ultra Warp Ride. They won't show up in your Pokedex either since there is no National Dex, and are a valuable way to get Pokemon otherwise not found in Ultra Sun & Ultra Moon. Here are what Pokemon you can get from each color of Ultra Wormhole excluding white, which has its own section following this one.
Blue wormholes will lead to a waterfall area. As such, you will find Pokemon that like water.
Red wormholes will lead to a cliffside area. Pokemon that love the clouds make their home here.
Green wormholes lead to an oddly desolate field/meadow area. Pokemon that like open areas live here, and it must be a gloomy place.
Finally, yellow wormholes lead to a dark cave. Pokemon who like to reside underground and away from the sun can be found here.