@kirsty91, what troubles me in this thread is the way that you frame your conversations with your partner -- the language of him "letting" you do things or the statement that he almost "made" you get rid of your cavy. Beyond the issue of your lonely pig, I am worried about you. As someone who was in an abusive relationship, I can tell you, no manner of reasonable negotiation is going to make an impact on someone who is determined to be in control. Abuse doesn't have to be physical. If he is threatening to take away a companion animal because the animal doesn't suit him, that is a controlling feature. And what starts as that kind of control can lead darker places.
You are right that just because you give 50% doesn't mean that you can have whatever you want. But at the same time, just because he gives 50% to the household, he doesn't get to make unilateral decisions either.
I can tell you that me "before" in the other relationship used the words "he let me" or "he says I can" and so forth a lot, and that it took me a long time to realize that a true partner would never create in me the need to use those sorts of words. With my husband, I can say "we decided" and "we chose," on matters that affect us as a couple. There is a difference between consulting and getting permission.
My husband doesn't particularly agree with or want everything that I want. But after the understandable moment of "argh, really?" he is the sort that will say, what am I really giving up, if I agree with my wife on this? And before I ask, I will think, what am I asking him to give up? In this case, getting a 2nd pig, it would be adding to the joy of your current pig and frankly as someone with 4 pigs, the 2nd, 3rd, and 4th didn't really add much labor- or cost-wise.
I hope for you that you can think, even beyond your piggy, and ask yourself "do I not deserve someone who thinks, how can I bring this person joy" and not settle for someone saying "how can I rein in this marvelous spirit and tell her that she can't do the right thing?"
The language of control starts quietly and only gets uglier. Count how many times a day that you say or think "he lets me..." or similar language. You deserve to be an equal partner of a person that truly regards you as equal.