Phil is the bulldog that Ren & Stimpy knew at the Dog Pound in the Pilot episode of the series. He was taken away by the Dogcatcher who put Phil to sleep. Or as Jasper described it as the "Big Sleep", which it's another word for "Death". Phil made one more appearance in the "Dog Show" episode alive and well.
At the end of the episode Big House Blues, Phil apparently was going to return and say, "Hi kids, remember me? I'm not dead! After all, this is a cartoon, isn't it?" Links to the storyboards can be seen here: https://drive.google.com/drive/folders/1y2IZt0TcQhfEohbHY_gQ7stZauOAVeEK