All first evolution stage pokemon except for legendaries can appear.
This is spread throughout the eggs as follows: You cannot get any further evolution stages of pokemon from an egg, nor can you get a legendary from an egg.
These distances were minimum distances at the start, however, that appears to have been fixed
gaming.stackexchange.com/users/20306/Unionhawk
2 KILOMETERS Bulbasaur Charmander Squirtle Caterpie Weedle Pidgey Rattata Spearow Zubat Geodude Magikarp Cleffa Igglybuff
5 KILOMETERS Ekans Sandshrew Nidoran(f)
Nidoran(m)
Vulpix Oddish Paras Venonat Meowth Psyduck Mankey Growlithe Poliwag Abra Machop Bellsprout Tentacool Ponyta Slowpoke Magnemite Farfetch'd Doduo Seel Grimer Shellder
Gastly Drowzee Krabby Voltorb Exeggute Cubone Lickitung Koffing Rhyhorn Tangela Kangaskhan Horsea Goldeen Staryu Mr. Mime Tauros Porygon Togepi Pichu 10 KILOMETERS Onix Hitmonlee Hitmonchan Chansey Scyther Pinsir Lapras Eevee Omanyte Kabuto Aerodactyl Snorlax Dratini Magby Smoochum Elekid
gaming.stackexchange.com/users/9892/oezi
I've found an updated version of what you get for every 2k, 5k, 7k and 10k egg.
You can also view the updated eggs on the Silph Road here
I plan to keep this answer updated when the eggs change.
Feb
1 2019 Taillow added to 2k eggs Lileep, Anorith, Feebas, Beldum, Nincada, Absol, Mawile removed from 7k eggs Jan 16 2019 Update
The eggs change so frequently that it's difficult to keep up with replacing the answer here.
So please refer to the Silph Road for the updated list.
gaming.stackexchange.com/users/135155/Hunter Turner
In conjunction to the other answers, it does appear that you can get 2km Pokémon from 5 or 10km eggs and 5km Pokémon from 10 km eggs.
A number of people have stated that they have gotten lower tier Pokémon from higher tier eggs.
Here are a few such examples.
A Mr. Mime from a 10 km egg.
( 1 , 2 , 3 , 4 )
A Pikachu from a 5 km egg.
( 1 ) A Magikarp from a 5 km egg.
( 1 ) A Magikarp from a 10 km egg.
( 1 ) A Bulbasuar from a 10 km egg.
( 1 ) A Zubat from a 5 km egg.
( 1 , 2 )
A Weedle from a 10 km egg.
( 1 ) A Charmander from a 5 km egg.
( 1 ) A Nidoran from a 10 km egg.
( 1 ) Given that so many people have reported this happening, it appears that there are tiers of Pokémon from eggs, however lower tier Pokémon can be hatched from higher tier eggs.
This picture here depicts these tiers.
gaming.stackexchange.com/users/127901/Dragonrage
As of the Generation II release, the egg tiers have been changed up quite a bit.
A full list can be seen on this site .
Someone converted this chart to image format here : Notable Gen
I Pokémon Egg tier changes:
From 5km to 2km: Abra Bellsprout Diglett Ekans Exeggcute
Gastly Goldeen Krabby Machop Nidoran F/M Oddish Slowpoke Venonat From 10km to 5km: Elekid* Kabuto Lickitung Magby* Omanyte Onix Pinsir Porygon
Scyther Smoochum* Tangela Tyrogue* *
These are Gen II pre-evolutions that replaced their parents in the egg pools but also switched tiers
gaming.stackexchange.com/users/162442/Vemonus
This list is presented first in order of egg-hatching distance and then alphabetical order, made using the lists from SilphRoad and GamePress : 2 KM eggs Abra Aron Barboach Bellsprout Bidoof
Gastly Kricketot Luvdisc Machop Magikarp Misdreavus Nidoran (both genders)
Poocyhena Slowpoke Spheal
Starly Swablu Swinub Taillow Wailmer Zubat
5 KM eggs Anorith Bulbasaur Buneary Cacnea Carvanha Charmander Chikorita Chimchar Chinchou Croagunk Cyndaquil Duskull Eevee Electrike Gligar Growlithe
Horsea Houndour Lickitung Lileep Lotad Magnemite Meditite Mudkip Nosepass Onix Phanphy Pineco Pinsir Piplup Poliwag Ponyta Sandshrew
Scyther Seedot Shuckle Sneasel Snorunt Snover Squirtle Stunky Tangela
Torchic Totodile Treecko Turtwig Yanma
7 KM – this includes Alolan forms (minus Rattata), baby Pokemon, and a few other Hoenn Pokemon Absol Alolan Diglett Alolan Geodude Alolan Grimer Alolan Meowth Alolan Sandshrew Alolan Vuplix Anorith Azurill Beldum Budew Chingling Cleffa Elekid Feebas Igglybuff Lileep Magby Mantyke Mawile Munchlax Nincada Pichu Riolu Smoochum Togepi Tyrogue Wynaut
10 KM Aerodactyl Bagon Beldum Dratini Drifloon Feebas Larvitar Mareep Porygon Ralts Riolu Shinx Skorupi Slakoth Trapinch
Some Pokemon that can hatch from 7km eggs can also hatch from a 5km or 10km egg: they are Anorith & Lileep (5km) and Beldum & Riolu (7km).
gaming.stackexchange.com/users/219593/MBorg
I have different Pokémon eggs. One takes 10km to hatch, three take 5km, and the rest take 2km.  Which Pokémon can I get by hatching these eggs?
How does egg hatching distance relate to the hatched Pokémon?
gaming.stackexchange.com
<pokemon-go>
gaming.stackexchange.com/questions/274373
gaming.stackexchange.com/users/147939/CarlosOro
All first stage pokemon can appear. A Magikarp comes from a 5 km egg, a Bulbasar comes from a 10 km egg and a Weedle comes from a 10 km egg. Whereas all first evolution stage pokemon may appear, there are numerous detailed posts here that will cover your question.
User gives general reply to question. Users list the specific ones that appear