The Plague Dogs

The Plague Dogs is a 1982 British feature-length animated adaptation of the Richard Adams book by the same name. The two main characters are dogs that have escaped from a research facility. One of them is voiced by John Hurt.

opening scene:

You can watch the full film online at this link.

Moria gives it 5 out of 5 stars and calls it an "unsung masterpiece". Den of Geek says, "The eye to detail extends beyond just eye-catching visuals."

The NYT praises the visual style and says it's "distinguished by its unusually lifelike animation." Rotten Tomatoes has a 90% audience rating.