There are many similarities between Venonat and Butterfree - the similar colours, huge red eyes, antennas, their hands and even similar mouths look similar. Also the appearance between Metapod and Venomoth are similar, with their same shaped head, black eyes and even the number of arms are the same as Caterpie.
What does this all mean though? Well for some reason Nintendo decided to swap Venomoth with Butterfree at the last moment.
The main theory as to why they done this goes back to when the anime first started; Ash's second Pokémon was a Caterpie - as the creators had planned, however once it had reached its final evolution, it would not looks as cute and attractive as a big bug eyed butterfly. Therefore they decided to change the two around so that it would be easier for children to get attached to Caterpie's final evolution.
Also Butterfree is classed as being white coloured in the Pokédex, even though the main part of its body is purple.
Finally here's some trivia you may not have known:
1. Out of the first generation Bug Pokémon, Venomoth has the highest base Special Attack and ties with Scyther for the highest base HP.
2. Venomoth's name is simply a combination of venom and moth.
3. Butterfree's name is a combination of butterfly and free, which possibly refers to its ability to fly after being grounded for two evolutionary stages.
4. Catrpie was the first Pokémon to be seen evolving in the anime.
5. In Generation I, Venonat could not learn any damage dealing Poison moves at all even though it is a poison-type Pokémon.