Very cool resource!
I am not sure if this counts as an official different form, but early Chansey artwork had Chanseys with 3, 4, or 5 side hairs on each side, including the early Sugimori Chansey in my avatar. However, I have seen no official artwork with 4 or 5 side hairs since 2003. The one instance of a Chansey without 6 total side hairs (3 per side) in the TCG is in the Stage 1 box of Blissey from Aquapolis. Outside of the regular TCG, there is the My Deck Memo Card and Sealdass.
www.elitefourum.com/t/evidence-of-chanseys-with-4-or-5-side-hairs-instead-of-3/28642/1