When the "Radeon RX 490" found its way onto the AMD website it could be dismissed as little more than a typo, but now it's discovered that this card is also mentioned on a Sapphire support page.
We still don't know if this card is real or not. AMD specifically said the Polaris lineup is complete so perhaps this is something that will only be made by certain add-in board partners. TweakTown speculates the Radeon RX 490 could be a dual-GPU card:
Polaris 10 is the most powerful GPU in AMD's arsenal before it moves to Vega, so it won't be a faster Polaris GPU with more stream processors - but two P10 GPUs makes perfect sense. SAPPHIRE's website states it has 8GB of GDDR5, but that's the way dual-GPUs work - each GPU has the same framebuffer, with 16GB of GDDR5 in total - but 8GB per GPU.