I also chose not to kill Olin, good decision. Ersa is Erend's sister and to tame a new mount you must approach in a stealthy way and when you are close the option to tame appears, it is easier if you are hidden in the undergrowth, but frankly I hardly use a mount, because on foot you can find more resources that are necessary to accumulate to create ammunition of the strongest weapons that you will get, but if it is of any use to you on PS4 it is by pressing triangle.