Clarification question about the scenario Saxon Medium Cavalry special rule "end any turn adjacent to a Viking unit" - is that any turn of either side or just a Saxon turn ?
any = either Saxon or Viking turn.
If the Vikings run up to them, they dismount to fight at the end of the Viking turn. Until the end of that turn, they are treated as Medium cavalry.
It seems pretty clear the Vikings and Saxons used horses to move around the countryside, but dismounted to fight. No indication of any cavalry tactics or suitable cavalry equipment. The Welsh seemed to have some sort of cavalry, probably Light cavalry, likely as a remnant of the Roman period.