Pokemon GO Nests Writeup

Q: Nests?
A: The following can nest (current 2018-05-23):
Abra, Aipom, Bellsprout, Bulbasaur, Caterpie, Charmander, Clefairy, Cubone, Diglett, Doduo, Drowzee, Eevee, Ekans, Electabuzz, Exeggcute, Gastly, Geodude, Goldeen, Growlithe, Horsea, Jigglypuff, Jynx, Kabuto, Krabby, Machop, Magikarp, Magmar, Magnemite, Mankey, Meowth, NidoranF, NidoranM, Oddish, Omanyte, Onix, Paras, Pidgey, Pikachu, Pinsir, Poliwag, Ponyta, Psyduck, Rattata, Rhyhorn, Sandshrew, Scyther, Seel, Shellder, Slowpoke, Spearow, Squirtle, Staryu, Tentacool, Venonat, Voltorb, Vulpix, Weedle, Zubat, Chikorita, Chinchou, Cyndaquil, Dunsparce, Girafarig, Hoothoot, Hoppip, Houndour, Ledyba, Marill, Misdreavus, Murkrow, Natu, Qwilfish, Remoraid, Sentret, Shuckle, Slugma, Sneasel, Snubbull, Spinarak, Sunkern, Swinub, Teddiursa, Totodile, Wobbuffet, Wooper, Yanma, Aron, Baltoy, Barboach, Carvanha, Corphish, Duskull, Electrike, Gulpin, Luvdisc, Makuhita, Meditite, Mudkip, Nosepass, Numel, Poochyena, Roselia, Seedot, Shroomish, Shuppet, Skitty, Spheal, Spoink, Surskit, Swablu, Taillow, Torchic, Treecko, Wailmer, Whismur, Wingull, Wurmple, Zigzagoon

Which leaves the following pokemon that cannot nest:
Aerodactyl, Anorith, Bagon, Beldum, Cacnea, Castform, Chansey, Chimecho, Corsola, Delibird, Ditto, Dratini, Farfetch'd, Feebas, Gligar, Grimer, Heracross, Hitmonchan, Hitmonlee, Hitmontop, Illumise, Kangaskhan, Koffing, Lapras, Larvitar, Lickitung, Lileep, Lotad, Lunatone, Mantine, Mareep, Miltank, Minun, Mr. Mime, Murkrow, Phanpy, Pineco, Plusle, Porygon, Ralts, Relicanth, Roselia, Sableye, Seviper, Skarmory, Slakoth, Snorlax, Snorunt, Solrock, Stantler, Sudowoodo, Tangela, Tauros, Togetic, Torkoal, Trapinch, Tropius, Unown, Volbeat, Zangoose

Consult the above table to find out which pokemon can or cannot nest. However, there are some rules that can be used to remember what does not nest:
- Pokemon in 10K eggs now or in the past
- Pokemon that are regional exclusive now or in the past
- Pokemon that gain an evolution in generation 4
- Pokemon with weather exclusive forms or are weather exclusive
- Pokemon that are holiday exclusive

Where the remaining pokemon that don't nest fit into none of these rules:
   Hitmonlee, Hitmonchan, Hitmontop, Grimer, Koffing, Stantler, Phanpy, Togetic, Ditto, Unown

Q: Should I visit a rattata / pidgey / yanma / etc. nest if I want a ditto?
A: Yes. Ditto doesn't spawn, per se, just Pokemon that can be ditto have a small chance to turn into ditto, and nest spawns are no exception.

This was an open question for some time that was closed when yanma could become ditto and yanma nests appeared in biomes that do not support yanma.