Seekers of Slaanesh
Seekers are Daemonettes that have shown promise in some way, and been allowed to the fields where Steeds roam to select and train a mount. The lucky Daemonette finds itself in good company, as the "playful" games of the Seekers are far more thrilling than those of ordinary Daemonettes. Gathering in bands of merry riders, they select an extraordinarily unfortunate soul and pursue them across reality wherever they go, just fast enough to stay on their heels before suddenly vanishing at the point of exhaustion for the victim and playing as if they have found them again when it can continue on. They continue to do this however long it takes until the victim goes insane from fear, be it days or years, and they finally surrender. The riders are inevitably disappointed that the game has come to an end and slay them to take their soul back to the Palace of Pleasure for an eternity as a torture victim for the ever-growing number of Daemonettes and Daemon Princes within.
On The Tabletop
Steeds of Slaanesh are the fast cavalry option for the Daemons of Chaos army. While they aren't as useful as Fiends of Slaanesh as hard-hitters in a monotheistic army, they require less points to get the full use making them a good budget investment.
Now called "Mounted Sirens", Steeds are now a balanced option and are something to be taken alongside Fiends rather than an alternative choice. Due to T9A Daemons giving greater incentives to monotheistic armies, players playing a "Lust" AKA Slaaneshi army list will invariably take them. Players going for a mixed army still will want to choose between them or Fiends.