We've known for a while that some Pokemon in Pokemon Go are a lot more common than others. However, charting the frequency of egg hatches are a lot harder to track, in part because it took time (and steps) to collect data. Eventually, players figured out that Pokemon Go realized that some Pokemon tended to hatch more than others.
The Silph Road, a Reddit community dedicated to researching Pokemon Go, discovered that it was easier to obtain some Pokemon from eggs than others, regardless of what type of egg they came from. Players were more likely to obtain an egg with a "common" Pokemon than they would an egg with an "uncommon" Pokemon. In total, there are four different tiers of rarity: common, uncommon, rare, and ultra-rare, each of which spawned at its own rate. Players typically got an "ultra-rare" Pokemon only once in every 315 eggs.
Yesterday, the Silph Road updated their findings to include the new Pokemon from Pokemon Go's recent Gen 2 update. Per the website, here are how rare each Pokemon hatch from eggs:
Common (hatch rate of 8/315): Ekans, Sandshrew, Nidoran (male and female), Zubat, Oddish, Paras, Meowth, Psyduck, Poliwag, Geodude, Ponyta, Krabby, Goldeen, Staryu, Dratini, Natu, Wooper, Pineco, Phanphy
Uncommon (hatch rate of 4/315): Bulbasaur, Charmander, Squirtle, Vulpix, Oddish, Diglett, Growlithe, Abra, Machop, Slowpoke, Magnemite, Shellder, Gastly, Drowsee, Voltorb, Exeggcute, Cubone, Rhydon, Scyther, Eevee, Pichu, Cleffa, Igglybuff, Togepi, Aipom, Gilgar, Slugma, Mantine, Stantler, Tyrogue, Smoochum, Elekid, Magby, Larvitar
Rare: (hatch rate of 2/315): Seel, Onix, Chansey, Tangela, Pinsir, Mareep, Sudowoodo, Remoraid
Ultra-Rare: (hatch rate of 1/315): Grimer, Lickitung, Koffing, Lapras, Porygon, Omanyte, Kabuto, Aerodactyl, Snorlax, Yanma, Misdreaveus, Wobbuffet, Girafarig, Dunsparce, Qwilfish, Shuckle, Sneasel, Skarmory, Miltank
The Silph Road noted that some of the "ultra-rare" Pokemon might actually be rare Pokemon, as their data pool was still small enough for some errors. Still, this new information is pretty useful for understanding how eggs work in Pokemon Go.