star trek - Why are there no non-humanoid Borg?

You rarely see non-human Borg, let alone non humanoid Borg. Are there any in-universe examples of non humanoid Borg, or do they only assimilate humanoid-type races (which would seem a rather limited way of gaining new technological distinctiveness-es)?

They certainly wanted to branch out, as species 8472 demonstrate, but we don't see any Borg drones of, say, shape shifters (and they would certainly be a very advantageous race to assimilate, though quite how Borg implants would work on them is another question entirely).

Answer

In the episode The Chase (TNG S6:E20), the primary reason for the expansion of life can be assumed to have been created by the proto humanoid species that seeded the galaxy many centuries if not millenia beforehand. This not only explains the existence of so many humanoid species in the Star Trek universe, but also the direct relation between alien species as descendants from a single origin. In all probability the Borg could assimilate other non-humanoids such as non-humanoid Xindi, it's just that the galaxy is primarily populated with humanoids.

The Borg could also possibly see the adaptation of there technology to work on non-humanoids such as Xindi-Insectoids and Xindi-Aquatics as both a significant drain on resources and at times not as useful, as in the case of Xindi-Aquatics.

http://en.wikipedia.org/wiki/The_Chase_%28TNG_episode%29

http://en.memory-alpha.org/wiki/The_Chase_%28episode%29

http://en.memory-alpha.org/wiki/Xindi

Expanding and correcting parts of my original answer there is also plenty of evidence of Borg assimilation of non-humans, but not non-humanoids. We can see part of the evidence of other alien humanoid species that have been assimilated with the episodes about Unimatrix Zero.

http://en.memory-alpha.org/wiki/Unimatrix_Zero

In the end it stands that there is only evidence of the Borg attempting to assimilate one non-humanoid creature, Species 8472. To me this is evidence that the Borg was only interested in advanced non-humanoids and would not concern itself with the relatively unuseful species such as Xindi-Aquatics.

I believe further evidence of this can be seen over here, in this answer.

Why aren't we all Borg?