Σ:3
Well according to Memory Alpha, the source for all things Trekkie:
http://memory-alpha.org/en/wiki/Assimilation
>Borg assimilation was predatory; every species the Borg encountered was assessed to determine whether its biological and technological distinctiveness was considered "worthy of being added to the Collective's perfection". If found to be so, the species was set upon and forcibly assimilated; the Collective considered the species' will in the matter "irrelevant".
Apparantly cats weren't considered worthy. :(