https://wccftech.com/assassins-creed-odyssey-mod-horse/

Assassin’s Creed Odyssey is one of the most complex entries in the series, having fully embraced role-playing game mechanics and made a central part of the experience. If you find some of these mechanics too limiting, however, you can now tweak them with a brand new mod.

The Assassin’s Creed Odyssey Tweak Pack is a lightweight tool which allows players to disable horse speed restriction, add a custom XP multiplayer to avoid excessive grinding and more.