While there may be plenty of battery pack cases on the market for the iPhone X, but the company known for their battery packs is Mophie. So far, unfortunately, they haven't delivered anything in regards to a battery pack for the iPhone X, except for wireless charging bases.
This might be changing pretty soon though. A product listing on Wireless Power Consortium (via Apple Insider) appeared for a new iPhone X case from Mophie. This new case will be part of the Juice Pack Air offering, and it looks just like Mophie's wireless charging case for the iPhone 7 Plus, but with different camera hole.
The case will have a 1,720mAh battery that will supposedly add none additional hours of “talk time.” According to the listing, it will only support Qi version 1.1.2 and the 5 watt Basic Power Profile. This basically means that you won't be able to take advantage of fast charging with the case.
There is something we should discuss though. The case is classified as a receiver, not a transmitter. There are some cases on the market for the iPhone X that charge the phone wirelessly, but aren't very efficient. Mophie may be a little late to the party, but better late than never, right?