Please please please please don't get/have a dog if you're not going to have someone to care for it when you're out of the house for 8++ hours during clinical years, or during pre-clinical if you have required lectures all day. I would love to come home every day to a dog, and it would have been perfectly feasible in MS1/MS2 as I didn't go to class, but that dog would be horribly neglected now that I'm a 3rd year and out of my apartment for 12 hours a day. Most of my classmates with dogs have significant others who are not in medicine who are able to care for the dog, at the very least let it out to pee in the middle of the day. On the other hand, one of my classmates now has to pay an arm and a leg for doggy daycare because it's not fair to leave the dog alone in an apartment all day.
Cats are fine. I'd have a cat if I wasn't so allergic.