I recently got the GBA4iOS Emulator, and got Pokémon Yellow to have fun with it. I love Pokémon and have played many of the games, played TCG, watched the anime, etc., But I have never played Yellow. I had seen a Let's Play on YouTube, but aside from that, I know virtually nothing about 1st Gen. (not counting Pokémon stats and stuff). When I got through Viridian Forest, I realized that Pikachu would not be enugh to beat Brock. Duh. I went and caught 2 Pidgeys, just to have some stuff to fight with, and I started to level them up. I got one to level 10, and then found a NidoranM. I thought that was cool, I caught it, and released my lower level Pidgey (cruel, but I didn't need 2 of them :frown:). I decided to train back in Viridian Forest because NidoranM was level 6. But then I found this. As you can see, I found a level 10 Pidgeotto, and I don't know if that happens, or if it's a glitch in the ROM (from doperoms.com), but lolwut? I don't think that basic Pokémon evolve until at least level 16, and I had a higher level Pidgey. Don't know if this has happened before but really, what is this?
In Pokémon Yellow you can find underlevelled Pidgeotto in Viridian Forest. It's intentional. That's pretty much all.
Yup I actually remember catching such a Pidgeotto in my first ever playthrough of Yellow when I was nine.
Spoiler In all seriousness, some of the early Pokemon main games used to have pokemon available at a level below the one they should be through evolutionary methods. You can read about it here.
I remember in Diamond one of the trainers by the Bicycle Road (I believe the southernmost adjacent route connecting it to Mt. Coronet) had an underleveled Bibarel. Toys R Us also gave away a Level 50 Dragonite. From the Bulbapedia page: This Dragonite was distributed at Toys "R" Us stores in the United States and Puerto Rico from November 8 to 9, 2008 from noon to 4 p.m. It was specifically released at level 50 because tournaments require Pokémon to be level 50 to enter, but Dragonite evolves at level 55 so is otherwise ineligible. Another set of notable examples are Falkner in GSC and HGSS, and Ghetsis' Hydreigon in Black and White. In GSC, Falkner's Pidgeotto is Level 9, and in HGSS, Pidgeotto is Level 13 (bear in mind Pidgeotto evolves at Level 18). Hydreigon, in the original Black and White, is Level 54, while Hydreigon evolves at Level 64.
Lance had a level 40 Dragonite in HG/SS. Game Freak sure doesn't mind bending the evolution rules I guess.