I've recently become interested in animal play as a type of objectification and have been discussing this with both Ethical Hedonist and The Photographer. Cats and dogs have never really appealed as it isn't precisely pet-play that excites me, but rather the dehumanising effect of being treated as an animal. As opposed pure objectification the submissive-as-animal has an identity with likes and dislikes, indeed, these whims of behaviour form part of the charm for both playing the role and being played with.
The Photographer had a switch related crisis as to whether he wanted to have a pony or be a pony and so our discussing flitted from both sides of the proverbial fence, which was helpful. Appart from the aesthetics and the kit involved, which are both hugely fascinating and attractive, ponies also operate in a very specific arena of play that makes me eager to try it out. They are trained and trainable but naturally headstrong; they are outdoor animals and beasts of burden. This gives them a certain focus on physicality rather than the emotional connection that might be more involved in puppies or kittens and a purpose beyond being entertaining or pleasing.
In all aspects of objectification I find myself focusing on the use-value of the object, that the connection to the Dom(me) lies in what it does, in achieving something, even if that achievement is in total passivity. That part of my pleasure derives from success in a definable context and that to be pleasing I must pass a certain test.