You can't.
Horsea evolves into Seadra at level 32. Seadra evolves into Kingdra when it is holding a dragon scale and traded. You can find dragon scales as a held item of wild Bagon in the cave behind the waterfall in RSE.
go to pokemon box, find the pokemon you want to evolve and click the green button that says evolve
seal,seeking,the evolve of seeking,a Pokemon in a shell that has horns,and dratini wow. you sure know your Pokemon (I'm being sarcastic). SeAking doesn't evolve, but the pre evolution is goldeen. the Pokemon with horns!?! WOW (sarcastic, again). That Pokemon is either Shellder or Cloyster. But by your definition, I'm not exactly sure! Oh, and it's not the evolve of seAking (which doesn't evolve), it's the EVOLUTION of seAking (which doesn't evolve). oh, yeah, and I capitalized the A in seAking, because it's not seEking! I'm sorry! I haven't actually played this game, but I just needed to correct these horrible mis-spellings. Hope this definition is more clear! Wow you need to chill. though the first person was kinda being an idiot. Sorry, but I just had too. Now I have played at least Pokemon Crystal, and it might be different in Pokemon Leaf Green and Fire, but in Crystal when you travel to the Kanto region she has a Staryu and a Starmie. Hope this improved definition helps! (P.S. I am the person who corrected this answer the first time. A.K.A. The second answer) 8)
kabutops
Aerodactyl does not evolve.
you need to trade it and it will evolve
sorry doesn't evolve in leaf green
Charmelon evolve on leaf green at level 36
Krabby will evolve into Kingler at level 28.
I don't think snasel can evolve into weavile in leaf green
it will evolve into LEDIAN at lvl 18 in pokemon Green leaf..
level 36.