Have you ever noticed that Spinarak the "Spider Pokemon"has six legs?For proof play colo.When you faint the Spinarak it goes onto its back...and then in full view you can clearly see six legs!
we call daddy-long legs a spider yet two of it's legs are acturally anttena... so it only has six legs...
Well, Ariados has those two antannae-like protrusions jutting out its back so, maybe it's based off the daddy-long legs!
Spinarak doesn't even have spider in the name. It has spin though.
I had the same thought about Perap since parrots are zygodactyl (two toes in front, two toes in back), but Perap has three in front and one in back.