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

Sahih Muslim 700a
Arabic hadith text
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَيُصَلِّي سُبْحَتَهُ حَيْثُمَا تَوَجَّهَتْ بِهِ نَاقَتُهُ ‏.‏
Translation

Ibn 'Umar reported that the Messenger of Allah (ﷺ) used to say Nafl prayer on (the back of) his camel in whatever direction it took him.