Just to be sure I'm assuming correctly, when it comes to advancing Epona with Adventure Experience.
Grogs and Companions default to 20 xp per year, which can be broken down into four 5-point seasons. In seasons where they adventure, receive training, or are taught, they can take one of those 5-point blocks and trade it for what they get with adventure/training/teaching.
So, in Epona's case, she would get 15 xp for 1237, plus the eight points for the adventure?
(I couldn't find it stated anywhere in the House Rules thread, the Experience wiki page, or the Character Creation page.)