I remember it was a short animation, though I saw it in a cinema… decades ago. As far as I can remember no human was visible in the whole story, in the beginning you see an island with some kind of animals chewing their lunch, when a flying object (whether it was an airplane or a saucer, I don’t know anymore), they beaming up some of the animals, taking them to a big town, making some experiments (hence: they put them into a vertical pipe, irradiate them) which turns them to monsters. The monsters are getting furious and break out, trying to get back to the island where they are from. On the way they fight with their kidnappers, and only one survives. As he (is) getting back on the island, he is turning back to the animal he was before.