This letter is for when you need to let your loved one know that you can't stand his or her dog, cat, or other animal friend.
You are the most wonderful thing that's come into my life in a long time. As I think you know by now, I love you very much. So it pains me to say this, but I must: I hate your <kind of animal>, <pet name>. <He/She> is always <pet behavior> which I really can't stand. I'm not much of a <kind of animal> person anyway, and <pet name> really rubs me the wrong way. I know you might take this personally, but I felt it was important that I let you know this now. Let's talk about how we can solve this problem. I'm sure there's a way short of euthanasia.