I have located milkweed in my area, southeast Id./northern Ut., that resemble antelope horn milkweeds, but have beautiful purple flowers and a darker green leave around them. They do not grow in swampy areas, but in dry sidehills or disturbed areas. Are they another variety of the antelope horn? I gathered a few seeds from the plants when the kids opened. Will they die if they are stored at room temperature, and do they need cold stratification.
They are likely a different species of Milkweed.
Here is a link that may help you with identification.