I've seen some similar things. If you check the book in this thread and search "barbarian cavalry" (no quotation marks), it looks like Alexander was phasing out the sarissaphoroi in favor of missile-armed, faster scout cav units:
http://www.twcenter.net/forums/showthread.php?t=148124 (you can't search "sarissaphoroi" 'cause the author uses Greek--I was quite shocked when I realized I could read it)
Wien1938: That unit looks a little heavy. Shouldn't they be more like prodromoi, with a core of hetairoi just like now?