Caterpie-Metapod-Venomoth?

If you look at Venonat and Butterfree, you'll notice that they share a number of similarities. Their eyes are big, red, and compound. They are both purple, and their antennae, mouths, feet, and tiny, claw-like hands resemble each other.

If you look at Metapod and Venomoth, you'll also spot similarities (though not as many as with Venonat and Butterfree). All three sport some form of pointed forehead structure and no antennae. Rather than compound eyes, all three pokemon posses mammalian sclera and pupils. Caterpie and Venomoth both posses four small feet on their upper thorax rather than distinct feet and hands. Venomoth's abdomen appears to have segments which may resemble the segments of Caterpie's body. The red protrusion on Caterpie's head is based on a defense mechanism possessed by some caterpillars in which they push one of their organs out of their mouths to mimic the tongue of a snake. Only swallowtail caterpillars are known to posses this ability and the protrusions on Venomoth's wings resemble the characteristic "swallowtail" for which these butterflies are named. Butterfree lacks this feature of its wings.

The theory is that in the first game, the editors messed up somehow and switched the sprites for Venomoth and Butterfree by accident. They later discovered the error after release and to cover up their mistake, they decided to leave the error rather than fixing it.