Their relationship progressed on and off for a few years until it reached it’s peak. Trixie wanted to be dominated by a stallion, and not just any stallion, but Rarity in the form of one. The full spell was pricy and only used to allow same sex couples to reproduce, so they opted for the black market potion version. This potion only changed ones genitalia, and rarely lead to pregnancies due to the low chance of producing viable sperm. As she often did, Trixie bid Rarity farewell to tour Equestria in her caravan. All was well, until Trixie returned with a foal.