Pongu and Nessie-the-foster-dog did not get along. This was 100% completely Pongu's fault. Nessie was never anything but a saint toward him, but Pongu could not and would not countenance another dog staying in his home and taking up the attention of his person.
He never got violent about it. Pongu was much more devious than that (plus he knew that being snippy would get him in trouble). But every waking moment that Nessie was in the house, Pongu was furiously plotting her downfall.
(Of course, we don't really know what he was thinking. There's considerable dispute over whether dogs can even feel so-called "complex" emotions like jealousy. But if you lived with these two dogs for more than fifteen minutes, I'm pretty sure you'd have to agree that, at the very least, Pongu was clearly capable of jealousy. And his expressions certainly suggested fiendish plotting.)
One afternoon, while the Spousal Unit was in the shower, Pongu stood at the bathroom door and began whining in his best "come-quick-Timmy's-in-the-well!" Lassie impression.
Pongu never does this, so the Spousal Unit was duly concerned and hurried out to see what was the matter. Perhaps the stove was on fire! Or the freezer had malfunctioned and was leaking poisonous goo all over the floor!
Helpfully Pongu trotted toward the living room, leading SU to... the nice comfortable white couch. Where the dogs are not allowed. And where Nessie was sprawled out full length, relaxing.
And at once Pongu turned to SU, his expression clearly conveying: "See? SEE?! This dog is breaking the rules! GET RID OF HER!!"