How do you get weimaraner's on Nintendogs Dalmatian?

First, find the Jack Russel book on a walk. This unlocks the Jack Russel Terrier in the kennel. On a walk with your new Jack Russel, it might find a Handkerchief, which unlocks the Weimaraner.

*Note: I have heard this is extremely hard to do, and the Handkerchief can only be found by a Jack Russel.