Dear Kate,
I have a dog and horse. My dog is a german shepherd and my horse is andalusian. I think that if you have a dog it might get lonely when youre away, for example in school or at work. With horses its much more complicated because horses are big animals that need to be taken care of. Horses also get lonely, they need lots of food, toys, water and many places run around.
I think a cat would be a perfect choice of animal for you because cats can live on their own. Youre always busy so the cat will go for walks and it will not bother you.
Kind regards,
Sandra