There’s a post that goes around Tumblr periodically full of anecdotes about horses killing and eating other animals, and even humans. I’ve been trying to track down any hard sources on this; the only one linked in the post itself is here. None of those anecdotes are themselves sourced.
A bit more digging came up with this post, which overlaps in content quite a bit with the Horseaholic post, but not completely. Both mention Shackleton’s ponies that he took on his voyage to the South Pole. It seems reasonable that in that environment, any source of calories would be utilized.
I also found this collection of modern-day anecdotes. Overall, I think including mention of feeding horses salted fish in an environment with limited resources (such as Iceland, as cited there) is reasonable, as is using meat in an extreme resource-restricted situation. In other cases, opportunistic meat-eating can be used as a quirk or notable event.
Lisette, the French mare who killed a Russian officer per the first two posts, does not seem to come up anywhere else that I could find. But it could be very useful as a military myth to weave into a culture in fiction.
