The thestrals. As long as they were fed, Snape wasn't unduly concerned. They were very good at looking after themselves, and the ones who'd been trained to pull the carriages understood their one day of duty. He'd look in on them on his not infrequent trips to the Forest for ingredients.
"Mr. Lawson,
Regarding the thestrals, I appreciate your honesty. As long as you keep them well fed, that will suffice. I will take other measures to monitor their wellbeing.
Dogs are not permitted within the Castle, or the outbuildings such as the Greenhouses or the Owlery, under any circumstances. Dogs are an unhappy combination with delicate plants and sensitive owls, not to mention all the cats students bring to Hogwarts.
However, as long as you keep your dog confined to the yard of the Groundskeeper's hut and the Forest, I am willing to overlook its presence on the grounds. Do keep in mind that if your dog is injured in the Forest or elsewhere, that is your own responsibility.
Sincerely,
Severus Snape, Headmaster of Hogwarts School of Witchcraft and Wizardry."