Plague Dogs (1982) - Rosen
A black retriever named Rowf and a fox terrier, Snitter escape from a top secret laboratory in a mountainous English town by the lake. After many water endurance test where many fellow lab dogs drown, Rowf has issues with water. Snitter has been subjected to brain surgery and has a hole in his skull and also has a lousy luck with humans (his masters die). To avoid starvation, they stalk sheep with the help of a snarky fox named Todd. The body count rises. The towns people are after them, the labcoats are after them. Soon the Defense Ministry is called in with paratroopers to smoke the dogs out and destroy them for fear of them carrying bubonic plague.
No less devastating than Rosen's Watership Down, Plague Dogs is nevertheless beautifully drawn animation with a strong anti-animal lab testing message. The ending will break your heart.