In season 4, Ripley is the dog who was ordered by Andrew to kill Seth.
The dog turned out to actually be very sweet so Brennan started growing closer with Ripley. Towards the end of the episode Brennan goes into Booth's office to tell him that she decided to adopt Ripley. While showing Booth all the things she got for her new furry friend Booth tells Brennan that they had to put Ripley down.