Pineco to Forretress
Nuzleaf to Shiftry
Whismur to Loudred
Barboach to Whiscash
Burmy to Mothim
That's my list for the 1st 4 generations, at least the ones that have not been said thus far.
I'm still not certain what the heck Forretress is, what a Bagworm is, but hey, they're similar in looks.... <.<
Nuzleaf to Shiftry...I don't know, just seems like a drastic change like Wotter's 2nd form to Daikenki
Whismur to Loudred...really drastic change in general appearance.
Barboach to Whiscash. 3rd generation, I found a Barboach in the mud, and I was like, what the fudge is this? What is this sea-weenie? To this day, Barboach is still the "Sea-weenie" Pokemon in my book. Then it turns into this bulky 90% Swampert.
Burmy, you rarely see him without a cloak unless you watch the anime, but that little black thing doesn't look like it can evolve into a moth.
Oh yeah, Luvdisc to Mamanbou, can't see the resemblence their either...wait...they aren't related