*[Cruel][1]* is possibly the most often used adjective for this context, though from my experience *cruelty* (as a noun) is more common.  (E.g. [SPCA][2] stands for the Society for the Prevention of Cruelty to Animals.)  The Wikipedia article is called *[Cruelty to animals][3]* and says that it is also called *animal abuse* or *animal neglect*.

Though there is no such word used exclusively for animals, we are not much the worse for the lack, since context will dictate the meaning of any word we choose when describing cruelty to animals.  For instance, if someone spoke of "inhumane treatment of animals", we would not think he meant "not suited for human beings"; we would understand that he was referring to cruelty to animals.  This shows that many synonyms of *cruel/inhumane* can be used and understood properly.

It's worth noting that even if the word *inhumane* is not commonly used to describe cruelty to animals, *humane* appears to be most often used as its antonym, as in the *[Humane League][4]*, *[Humane Society][5]*, or *[humane treatment][6]*.


  [1]: http://dictionary.reference.com/browse/cruel
  [2]: http://en.wikipedia.org/wiki/Society_for_the_Prevention_of_Cruelty_to_Animals
  [3]: http://en.wikipedia.org/wiki/Cruelty_to_animals
  [4]: http://www.thehumaneleague.com/
  [5]: http://www.humanesociety.org/
  [6]: http://www.google.com/search?gcx=c&sourceid=chrome&ie=UTF-8&q=humane%20treatment