Route 2 Grotto: Nidoran (F/M), Granbull, Watchog
Route 3 Grotto (Tall grass): Manectric, Pachirisu, Zebistrika
Route 3 Grotto (Pond): Venonat, Lombre, Bibarel
Route 5 Grotto: Minccino, Liepard, Foongus
Route 6 Grotto (Near Breeder): Dunsparce, Woobat, Foongus
Route 6 Grotto (Mistralton Cave): Nosepass, Woobat, Foongus
Route 7 Grotto: Zangoose, Seviper, Watchog, Cubchoo
Route 9 Grotto: Liepard, Muk, Garbodor, Bouffalant
Route 13 Grotto (Stairs): Spheal, Drifloon, Foongus
Route 13 Grotto (Giant Chasm): Tangela, Skorupi, Foongus
Route 18 Grotto: Kingler, Dragonite, Corsola, Chatot
Route 22 Grotto: Pelipper, Amoonguss, Mienfoo
Route 23 Grotto: Golduck, Gligar, Zangoose, Seviper, Absol
Abundant Shrine Grotto (Near Youngster): Vulpix, Bronzor, Amoonguss
Abundant Shrine Grotto (Shrine): Golduck, Swabloo, Amoonguss
Giant Chasm Grotto: Clefairy, Ditto, Sneasal, Metang
Lostlorn Forest Grotto: Pinsir, Heracross, Combee, Leavanny
Pinwheel Forest Grotto (Outer): Poliwhirl, Bagon, Hariyama, Medicham
Pinwheel Forest Grotto (Inner): Butterfree, Beedrill, Amoonguss, Murkrow, Breloom
Floccesy Ranch Grotto: Marill, Dunspare, Herdier
Funfest Mission Grotto: Glameow, Stunky
Also, you will not always find Pokemon in Grottos, you could very well find an item (e.g. an evolutionary stone). Here is list of items found it Hidden Grottos.
Source