The thing with Zelcor’s riding dogs wasn’t planned at all; the dogs kept dying, as could easily be predicted, and Zelcor’s player started telling me how the various animal sellers were getting mad him for losing so many dogs. At this point Zelcor probably doesn’t even need a riding dog, but he keeps buying (and losing) them because we find it hilarious.
The other point to be made about the riding dog situation is that it illustrates the benefits of a long-running campaign that isn’t super lethal. PCs can reasonably expect to have long adventuring careers, and because of that have time to develop various quirks and character traits that greatly enrich the game. Not every character develops like this, of course, but if you as GM are killing off characters left and right you’re removing the opportunity for anyone to do so.