The context of this verse:
The wolf will live with the lamb, the leopard will lie down with the goat, the calf and the lion and the yearling together; and a little child will lead them. The cow will feed with the bear, their young will lie down together, and the lion will eat straw like the ox. The infant will play near the cobra’s den, and the young child will put its hand into the viper’s nest. They will neither harm nor destroy on all my holy mountain, for the earth will be filled with the knowledge of the Lord as the waters cover the sea. (Isaiah 11:6-9 NIV)
Is not talking about animals, it is talking about personalities. I will attempt to De-Analogize.
The viscous will live with the humble, the secretive and shrew will lie down with the pushy, the vigorous and the domineering and the full of life together; and a little child will lead them. The content will eat with the anxious, their young will lie down together, and the domineering will eat bread like the worker. The infant will play near dangerous places, and the child will put its hand into dangerous homes. They will neither harm nor destroy on all my holy mountain, for the earth will be filled with the knowledge of the Lord as the waters cover the sea. (Isaiah 11:6-9 De-Analogized)
But to answer the attitude of animals before and after fall, this I do not know. Except for that of man, that I do know. Adam was not depressed until he fell into depression because of the knowledge of good and evil.
Psychology helps to understand this with the stages of grief, after denial caused from knowing good or evil we fall to anger, bargaining, and depression (self anger), but through forgiveness offered by the death of Jesus we can bring ourselves to acceptance and we can return to the vine (John 15). Faith being the opposite of denial saves us from depression.
If animals fall to depression I do not know, I have seen sad animals, but mourning and depression are two different things. So deductively this statement can be built.
If animals become depressed, they they also have fallen, else their spirit remains pure.
So if you yourself are depressed or angry, know your lack of faith. For many you can hear it happen with a sigh. So when you "can't believe it" just believe and you'll be cool.