AT&T is offering a limited-time deal for Next customers. Buy an eligible iPhone and get US$1000 in credit that you can use to buy an iPhone X. The credit will be spread out over the course of your service agreement up to 30 months. Eligible AT&T iPhone models include iPhone 8 and iPhone 8 Plus. You have to buy it on either an AT&T Next plan (30 mos. up to $38.34/mo. for iPhone X, $31.67/mo. for iPhone 8 Plus, $28.34/mo. for iPhone 8) or AT&T Next Every Year (24 mos. of $47.92/mo. for iPhone X, $39.59/mo. for iPhone 8Plus, $35.42/mo. for iPhone 8). Further details can be found here.
This is how you can get free iPhone X, iPhone 8, iPhone 8 Plus: Deal you can't miss
Apple's products that are eligible for AT&T's BOGO deal are the iPhone X, iPhone 8 and iPhone 8 Plus.