A dog requires a lot of time and money. What if you need to take your dog to the vet for an emergency and you have an exam that morning? Faculty will likely not be happy rescheduling a makeup for you. Be prepared to shell out a couple hundred/thousand in case of vet bills. Unless you have a spouse at home who works and can take care of the dog, I would not recommend it unless you had a support system at home to help with the dog. That being said, I had a dog during the end of school (D3/4 year) and she made my life 1000x better.