The Plague Dogs (1982)
"An animated adaptation of Richard Adams' novel, about a pair of dogs (Snitter and Rowf) who escape from a research laboratory and try to survive in the wild with the help of a cunning fox (The Tod). The lab director tries to keep the escape quiet, but as an increasing number of sheep are found killed, word leaks out, together with rumours that the dogs might be plague carriers..." -imdb.com
beautiful movie.