By trading only. These Pokemon are unobtainable by any other natural gameplay, unless you used the Mew Glitch, which you would need to find certain trainers to activate the Pokemon you would want to catch.
For more information on the Mew Glitch, just Google it or something, or go to Gamefaq's, and search the glitch guides, FAQ's, something.
The Pokémon that you cannot find in Pokémon Blue are Ekans and its evolution of Arbok, Oddish and its two evolutions of Gloom and Vileplume, Mankey and its evolution of Primeape, Growlithe and its evolution of Arcanine, Scyther and Electabuzz.
From http://www.gamewinners.com/game_boy_color/PokemonYellow.htm Missing Pokemon: Weedle, Kakuna, Beedrill, Ekans, Arbok, Meowth, Persain, Koffing, Weezing, Jynx, Electabuzz, and Magmar. From http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_Red_and_Blue#Version_exclusives Missing Pokemon (essentially the Red exclusives): Ekans, Arbok, Growlithe, Arcanine, Oddish, Gloom, Vileplume, Mankey, Primeape, Scyther, Electabuzz So between Blue and Yellow the only Pokemon you can't get are: Ekans, Arbok and Electabuzz
red: ekans, arbok, oddish, gloom, vileplume, mankey, primeape, growlithe, arcanine, scyther, electabuzz. blue: sanshrew, sandslash, vulpix, ninetales, meowth, persian, bellsprout, weepinbell, victreebel, magmar, pinsir.
Mankey is a pure Fighting-type Pokemon.
Mankey evolves at level 28.
you can find a mankey on route 1 and 2
Mankey evolves into Primeape at level 28.
You get mankey in Pokemon yellow by going to the grass on the way to the Pokemon league. It is going to be from level 3 to 5.
you trade it from another Pokemon gameyou can not get mankey on heart gold or soul silver yoU have to get him by migrating
Mankey evolves into Primeape at level 28
mankey evolves at level 30-35
Pokemon only in Red - Ekans/Arbok, Oddish/Gloom/Vileplume, Mankey/Primeape, Growlithe/Arcanine, Scyther, and Electrabuzz Pokemon only in Blue - Sandshrew/Sandslash, Vulpix/Ninetales, Meowth/Persian, Bellsprout/Weepinbell/Victreebel, Magmar, Pinsir