As I think that a pumpkin Pokemon is a perfect opportunity to make one based on the Headless Horseman, and as the only current horse Pokemon is a Fire-type, I'm sticking with Grass/Fire.
Also, a jalepeno Pokemon doesn't really interest me that much, and even if it did exist, it's not like there can only be one Grass/Fire Pokemon. I mean, the fifth gen introduced two completely unrelated Fire/Psychics.
Maybe it evolves when a Rapidash is in the party?
A possibility I have considered, though I think I first came up with the idea prior to the fourth gen, so I didn't know about Mantyke and, at the time, made it a stone evo like many other three-stage non-starter Grass-types (I called mine Jumpkin/Jackolite/Jackhollo - the last one because of the Headless Horseman connection, as he's from the Legend of Sleepy Hollow).
Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere