I have a divided opinion on this and so didn't vote, heh. I think if I have a personal relationship with the animal, i.e. my own, or a friend's, then it's strictly gender specific. And it would get on my nerves if someone continually referred to Mercy as a 'he'.

However, from my personal veterinary perspective, I would say it's fairly commonplace to refer to an animal as 'it', even if the gender is known. Probably more so in the farming and equine world, but also when discussing pets with other members of staff. I believe for courtesy's sake I always refer back to he/she when in the presence of the owner, as again, I know it would drive me batty were the roles reversed. I generally refer to the correct gender and 'it' pretty interchangeably at other times.

That's just my experience of shadowing vets in practice, at least, and it sort of seems the done thing. It most certainly isn't derogatory.