Would have been great if a last alliance of men and elves--um Andalites marched on Sauron--uh The One.
Oh that Katherine and her LotR influences
Does that make our beloved writer a FanGirl?
Also, I'm glad you bought this up, weathervane. I've been thinking this over for a while now, but like JFalcon said, there are a few species that refer to themselves by number in the Animorphs Universe. The first that come to mind are the Generationals from the Ellimist Chronicles who constantly revise their species name with each succesive generation (Generation 256 becomes Generation 257, etc).
To add on to your question:
[spoiler]It is possible that The Five fled Andalite persicution into Kelbrid space, maybe assimilating in to one singular form (The One) believing the collective mind/form to be stronger than an independent one.
Who knows? As Ax said, <Andalites in that era are not the Andalites of today.>
Maybe the Kelbrid are nothing more than a faction of Andalites that sided with the philosophies of The Five the same way that the Yoort are a seperate evolutionary path of the Yeerks. In that sense, they may have even altered their original form and followed an entirely different scientific approach (never discovering morphing technology) and used the Kelbrid/Andalite treaty to do so away from prying Andalite eyes while preventing the Andalites from realising that they actually hail from the same gene pool...[/spoiler]
But it's like K.A. is always saying. If it isn't included in the chapters or left obscure, it's usually left to reader interpretation/imagination. So, until K.A. writes about it, it means whatever you think it means.