I do believe it is okay to, in general, pray for our animals. Praying for their owners is always good, but I don't see anything wrong with having a prayer with the welfare of an animal being an end in itself. However, I am thoroughly convinced that they should not be put on the prayer roll. I don't know if I can really elaborate more than saying that I think it is made apparent in the temple that the prayer roll is intended for individual people, and not for anything else (such as worthy causes).