The Slavery Arc is an ongoing storyline involving the enslavement of Equestrians, among others, in the world of Rigel Prima. This arc has been slowly building since 2015 and involves many locations and characters. As of June 30th 2018 the plotline regarding the slavery arc has not resolved.
The first inkling of the slavery trade began with a scene involving sea grift Eridan Ampora and his crew known as the Sea Trolls capturing ponies in Town to take out to sea to sell. Eridan's attempts to kidnap the ponies on-screen had been thwarted by Town's elite police force the Ginyu Squad who proceeded to capture and arrest Eridan for attempted kidnapping. Eridan during his first incarceration indicated that this was not the first time he had kidnapped ponies to sell as slaves. Prior to a trial, Eridan busted out of Town jail with the help of his free crew and escaped Equestria before being brought to justice.
Eridan reappeared later to Barris Ki'Teer after pilfering from Valorcall and New Northumbria. He had the Cheshire Cat as one slave for sale. Barris bought the Cat off Eridan and left the rest of his stock. It's implied that the slave trade occurs at the Stormwall, notably in the lawless areas of the region. It's during this interaction that Eridan reveals using a device known as a 'Null Collar', an incredible device which can surpress supernatural abilities and magic when attached to the target destined for neutralization. This device was shown to completely inhibit the Cheshire Cat's many abilities, much to its chagrin. These same collars were also used on Unicorn and Pegasi Equestrians to limit their powers.
Finally, Baro Ki'Teer was shown to be part of an elite auction setting which included human slaves and taxidermied bodies of Equestrians and Animans.
After his second capture, this time by Wade Wilson's intervention in 2017, Eridan Ampora made a plea deal with the Equestrians to lessen his sentence to being exiled to the planet of Afteron rather than indefinite incarceration or worse on Rigel Prima. Eridan confessed that he sold slaves to a contact in Po-Koro's Saltshore, known only as 'Rakado'. Eridan detailed that he was not the only slaver to work the system and that the slavery circuit itself had 'deep roots' that could not be easily killed. He detailed that Rakado could be contacted but in very specific circumstances and could be spooked easily or driven into hiding. If that were to happen he was noted to be 'impossible to find' making it important that Rakado meet with someone he believed was trustworthy and had goods to sell.
However, Eridan's advice never went to use. The Equestrian government had not made any move against Po-Koro or the known lead Rakado after Eridan's capture. Following the cataclysmic event of Mo'Gallile and Rotan disappearing November 2017, Rakado stepped down from his position as a frontman of the slavery circuit to return to his home in the Allied Reeve Citystates of Rotan. He is credited for being flush with cash from the trade that he is using to rebuild the Reeve homeland, marking him as one of the most influential individuals following the tragedy.
As of now no more slavers have been revealed and it is unknown how far the trade goes or what countries condone it.