We don't know with any certainty just how massive or open the world of Horizon: Zero Dawn will be, but it seems it will be big enough to justify the use of mounts. In an interview with DualShockers, senior producer Mark Norris assured us that the protagonist, Aloy, will have other means of transportation at her disposal, remembering other open-world games like The Elder Scrolls V: Skyrim, The Witcher 3: Wild Hunt and even Grand Theft Auto V, all of which come with some sort of mount.
Mark Norris didn't specify what kind of transport or mounts Alloy will be able to ride, but he assured players "it's not just walking and running." Considering everything we've seen so far from the game, it would seem natural (and fantastic) if you can ride the cyber dinosaurs.
Horizon: Zero Dawn will be released in 2016 exclusively on PlayStation 4.