Hey, I've actually heard of that movie- haven't seen it though. For those that don't know, it's about two dogs who escape an experiment laboratory in England, but face adversity not only from their unfamiliar environment but the captors who are sent to retrieve them. I recall a movie guide said it was downbeat but well-done.
Oddly, I haven't seen many (if any) animated movies which are so downbeat that it's hard to rewatch them. I haven't seen End of Eva or Grave of the Fireflies, which are apparently some of the more depressing movies out there, regardless of their quality.