I've been wanting to add new locomotion styles to the Pets models for a few months now, but I can't find much on adding my own to the game files and having them work as intended. I have a horse gallop ready if anyone knows a possible way to do this.
Maybe you can add the animation with the same name as the original and the same fnv hash. That way it'll work like a tuning mod except you're replacing an Animation instead of an XML or ITUN file. I can't guarantee it'll work but it's worth trying.