Awesome topic. Hmmm one pokemon to embody each season.
Fall - *Waits for turkey pokemon.*
Winter - jellicentfan is on to something with abomasnow. It's a mix between the mythical yeti and a snow covered Evergreen Tree. The snow covered tree part specifically embodies winter while the yeti part shows some folklore behind what the season's know for - snow. You could make a case for Delibird, but that embodies Christmas not the entire winter season.
Spring - Beautifly. I think it's more of a butterfly than Butterfree is, and it's prettier. A butterfly fluttering around after emerging from its cocoon captures the rejuvenation of the land after winter.
Summer - Cherrim, it flourishes in the sun, blossoming whenever the sun's rays touch it. During the short summer nights it recedes under its petals. The sun is what makes summer, and Cherrim, special. Allowing people to come out an enjoy themselves.