>"Now that you have the SoulBadge, the Defense of your Pokémon increases! It also lets you Surf outside of battle! Ah! Take this too!"
>"TM06 contains Toxic! It is a secret technique over 400 years old!"
This is what Koga tells you after beating him in RBY. Although it's only implied, i've always thought of toxic as a man-made ninja technique. No pokémon could learn it by leveling up before gen III as well.
