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.
Leather
Leather is outlawed in Equestria, it’s considered a crime against equinity(pony humanity). The production of leather involves many horrible acts, from enslaving fellow sapient creatures to slaughtering those creatures for their meat and hides. Hides are processed into leather and meat is sold to dragons and griffons for consumption. Horns, bones, and organs not sold as food are turned into potion ingredients. All of this is very illegal.
Curse Effects
Rarity has a callous disregard for the lives taken to provide her with sexy leather clothing. Over the past few years Rarity has greatly increased the prices of her various ensembles, and knows just how to sweet talk 200 bits from your saddle bags. She keeps her best outfits for herself and overcharges ignorant ponies for simple items.
Trixie hasn’t changed much, she was never a very friendly pony.