I hope I haven't misinterpreted Boss's "サンドスターによってフレンズ化される種の区分はしばしば曖昧で" here, but it sounds like he's invoking the "rule" that there can be only one Friend per species at a time. Otherwise I don't see how the concurrent existence of Red, Ezo and Silver Fox would be a source of puzzlement.
Who?In the JungleThey're our friends from the Kyôshû Region!Could she be a close relative?The ezo red fox and silver fox are subspecies of the red fox.The boundaries between species transformed by Sandstar are often vague.There are still many mysteries.By the way, Red Fox,you look a lot like Ezo Red Fox and Silver Fox!