Animated movie. Animals escaping poisonous gas.
I would have seen this in the 90's. I have no idea when it was made. From what I remember the animals were living in the woods and escaping poisonous gas. I remember the gas moving/rolling through the woods, or possibly fields. Wherever it was, it was catching up to them. I think some of the animals died. Maybe all?
I watched the trailer for Once Upon a Forest https://www.youtube.com/watch?v=hLKuBn7_Clw
I don't think that's it. I remember the animation being prettier, and it having a more somber tone.
Any help appreciated!