It is permissible to offer voluntary prayers atop one’s mount when travelling, no matter what direction it is facing Sahih Muslim 700 e

Sahih Muslim 700e
Arabic hadith text
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ عَمْرِو بْنِ يَحْيَى الْمَازِنِيِّ، عَنْ سَعِيدِ بْنِ يَسَارٍ، عَنِ ابْنِ عُمَرَ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلميُصَلِّي عَلَى حِمَارٍ وَهُوَ مُوَجِّهٌ إِلَى خَيْبَرَ ‏.‏
Translation
Ibn 'Umar reported

I saw the Messenger of Allah (ﷺ) praying (Nafl prayer) on a donkey's back while his face was turned towards Khaibar.