1st Generation Pokemon Eggs
As in just about every other Pokemon Game, eggs are a key aspect in Pokemon GO. Outside of going out into the world and manually searching for a Pokemon, hatching Eggs is the second best way of finding both new and old Pokemon. Actually, Eggs even give a trainer a certain higher chance of finding Pokemon that don't normally spawn in their geographical area too often. Pokemon that hatch from Eggs usually have better Individual Values (IVs) than Pokemon found in the Wild.
There are three different types of eggs: 2km Eggs (Required to be Incubated for 2km & are the color Green), 5km Eggs (Required to be Incubated for 5km & are the color Orange), and 10km Eggs (Required to be Incubated for 10km & are the color Purple). (Only 1st Stage Evolution Pokemon Can Hatch From Eggs.) Eggs also give a trainer x100 Stardust for every kilometer the Eggs is, as well as plenty of Pokemon Candy. As more Pokemon are added to Pokemon GO, the Pokemon from each Generation that can be found in Eggs also changes to better balance things out. (egg Charts can also change temporarily during certain promotional events.)
1st generation pokemon egg Chart
1st generation pokemon Found In 2km eggs:
Nidoran ♀, Nidoran ♂, oddish, diglett, abra, machop, geodude, slowpoke, gastly, krabby, exeggcute
1st generation pokemon Found In 5km eggs:
bulbasaur, charmander, squirtle, growlithe, poliwag, ponyta, seel, grimer, shellder, onix, drowzee, voltorb, cubone, lickitung, koffing, Rhyhonr, tangela, scyther, pinsir, eevee, omanyte, kabuto
1st generation pokemon Found In 10km eggs:
chansey, lapras, porygon, aerodactyl, snorlax, dratini