For "reasons" I was looking at the Emishi list and recalled that the DBMM list only had them as horse archers from 700 CE noting that before then they are never referred to as riding horses but afterwards are noted horse archers.
Obviously, knowledge moves on and previous army lists are not always a good guide for what is current thinking, however, given this note almost certainly came from Duncan Head whose knowledge on this I would not doubt, and that so little is known of the Emishi that I wonder if the MeG list has its classification correct.
About 700 CE would also be fairly consistent with the increasing use of the horse in Japan which prior to that had been very limited indeed.
There's a bit of a puzzler in that the Yamato Era Japanese list, which has little cavalry, goes up to 790, and is superceded in 794 by the Heian, which has lots of cavalry. But it was the Yamato Era Japanese that started the 38 year war with the Emishi in 773. Since there are sources for that war but few of relevance previously it might be a case of "by 773 they had cavalry, we don't really know about earlier"
The Yamato list can have 20 bases of cavalry after 645 which is nearly as many as the Heian list (24).