What Happened to the Pet?

Bob has a dog. You see the dog greet him exuberantly when he comes home, he introduces the dog to Alice when she visits, the dog enters into a couple of episode plots. Then the dog gradually fades from view. Entire episodes take place in Bob's home with no sign of his dog. You begin to wonder if the dog has taken to hiding whenever other people are around. Have the writers forgotten that he has a dog?

Then, unexpectedly, they mention the dog. Apparently he ran away a long time ago, or maybe was killed. The Fridge Horror sets in as you realize that Bob didn't even mention, much less mourn, the loss of his pet.

Basically, this is when the disappearance of a pet from a show's plot is explained long after the fact by the pet's death or disappearance. To qualify, there must have been no comment at the time - the only explanation occurs much later, well after they should have shown a reaction.

Pet-specific Sub-Trope of What Happened to the Mouse?

Contrast Brother Chuck, in which no explanation is given.

Examples of What Happened to the Pet? include:

Literature

In the Tom Clancy novel Patriot Games, Jack got his daughter a Labrador named Ernie. It does not appear in any later book, though a comment in Red Rabbit, the next book in chronological (If not publication) order, implied that the dog stayed in America when the Ryan family moved to London for a year or so. No explanation is given for why they aren't reunited with the dog when they moved back to Annapolis.

Live Action TV

Kit the cat from Charmed apparently ran away at some point, and despite her being the Halliwell sisters' familiar, they didn't comment on it.
On Buffy the Vampire Slayer, Willow and Tara got a kitten, who was only seen in three episodes. An off-hand comment much later on suggests that this kitten was killed off-screen by an accident.

Dawn: I told you, I don’t leave crossbows around all willy-nilly… Not since that time with Miss Kitty Fantastico.

Joey and Chandler's duck and chicken from Friends. In the series finale, it is implied that they died and Joey was told that they were "sent to a farm".
On Wizards of Waverly Place Max at one point admits that everything about the the wizarding world is true, "except for the Dragon Dog! I have no idea what happened to him!" The episode the dragon dog appears the Russos jump through some serious hoops to keep that little puppy-dragon. And then he disappears. And then all we get is Max's lampshade hanging.