Alright. Here's what I have thus far.
The one that hasn't been colored is the Autumn version... What do you think of the spike/fan thing? The tail aside, there isn't a whole lot of interesting detail to the sprite, and this is one of the things I'm considering. Maybe I could try something more type-specific; flames, rocky projections, shards of ice... No ideas for the Spring form. Maybe smaller flower buds?
Just FYI, the Summer and Autumn forms will be more hostile-looking. I just wanted the sprites to match a snake's behavior in each season.