Thanks for the review. A large number of substances are poisonous in high enough quantity, including oxygen. Naturally, different animals have different sensitivities, which is why dogs shouldn't be given 'human food'- not because they don't deserve it but because it's not always safe for them. Toby is useful for Harry because, unlike Reggie, who is bidable, with simple needs, and Hedwig, who is pre-trained and independant, Toby a. requires training, b. has complex needs and c. doesn't understand what is good for him; like most dogs, he'd happily eat himself into an early grave. Therefore, Harry must learn to be responsible for someone other than himself.  

Thanks ^^ In the canon, although Harry often behaves much older than his years, he is also quite irresponsible; the events at the end of Order of the Phoenix are one example of a situation where a little consideration might have prevented a tragedy. Toby is going to teach Harry to be patient and the importance of considering all the posibilities of his actions, whether leaving a basket of chocolate on his bed or dashing into danger.