Island Scan Pokémon are the only Pokémon catchable in Sun/Moon that are not in the PokéDex. Here are all of them, plus their evolutions:
- Totodile (+ Croconaw, Feraligatr)
- Deino (+ Zweilous, Hydreigon)
- Horsea (+ Seadra, Kingrda)
- Klink (+ Klang, Klinklang)
- Litwick (+ Lampent, Chandelure)
- Cyndaquil (+ Quilava, Typhlosion)
- Spheal (+ Sealeo, Walrein)
- Luxio (+ Shinx, Luxray)
- Honedge (+ Doublade, Aegislash)
- Venipede (+ Whirlipede, Scolipede)
- Bellsprout (+ Weepinbel, Victreebel)
- Marill (+ Azurill, Azumarill)
- Gothita (+ Gothorita, Gothitelle)
- Swinub (+ Piloswine, Mamoswine)
- Duosion (+ Solosis, Reuniclus)
- Roselia (Budew, Roserade)
- Staravia (+ Starly, Staraptor)
- Vigoroth (+ Slakoth, Slaking)
- Axew (+ Fraxure, Haxorus)
- Rhyhorn (+ Rhydon, Rhyperior)
- Conkeldurr (+ Timburr, Gurrdurr)
- Togekiss (+ Togepi, Togetic)
- Leavanny (+ Sewaddle, Swadloon)
- Serperior (+ Snivy, Servine)
- Samurott (+ Oshawott, Dewott)
- Emboar (+ Tepig, Pignite)
- Eelektross (+ Tynamo, Eelektrik)
That is all that I could find. Zygarde is in the Alola PokéDex (#205). You could maybe count the RotomDex, because you have one in-game but Rotom is not in the PokéDex data, but you can't use him in battle so it is debatable.
If you count the Sun/Moon Demo, you can also get a Greninja. Credit to MJC1011.
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