It is a Priority Quest. You will automatically receive this quest after turning in the first quest from Act IV, to deliver Lorath’s letter to him.
Proress the campaign and complete “Mount: Donan’s Favor” to gain access to mounts.
Mounts are locked behind the priority quest “Mount: Donan’s Favor”. This Quest becomes available at the start of Act IV’s main story. You simply have to talk to the Stable Master in Kyovashad to receive it. After completion, you can use the mount and customize it with trophies and bardings.