We had 2 dogs, Jack and Floppy, they were together since they were 6 mos. old. Jack passed away at 15 and Floppy grieved for about 5 months. Because of her age, we really didn't think she would tolerate another dog well but she seemed kind of at odds with herself so we decided another furry body might be just the ticket and got a cat. We named him Buddy and although they are not close companions, they get along well. Buddy has to be outside if Floppy is outside and I notice he looks after her if he senses she is nervous. Floppy turned 18 this month and is doing well so I think I made a good decision.
She is not the same dog she was when Jack was alive though. Her habits have changed, and I found, in talking with other dog owners who had companion pets, that's not unusual when one companion dies.