Although some of the specifics are different, I think you are likely remembering the conclusion of Black Elk Speaks, which is presented as the as-told-to autobiography of Nicholas Black Elk, a spiritual leader of the Lakota nation.
I did not know then how much was ended. When I look back now from this
high hill of my old age, I can still see the butchered women and children lying
heaped and scattered all along the crooked gulch as plain as when I saw them
with eyes still young. And I can see that something else died there in the bloody
mud, and was buried in the blizzard. A people’s dream died there. It was a
beautiful dream.
And I, to whom so great a vision was given in my youth,—you see me now a
pitiful old man who has done nothing, for the nation’s hoop is broken and
scattered. There is no center any longer, and the sacred tree is dead.
http://public.gettysburg.edu/~franpe02/files/%5BJohn_G._Neihardt%5D_Black_Elk_Speaks__The_Complete_(z-lib.org).pdf
In context, the quote was indeed spoken as a reaction to Wounded Knee, and (currently) appears on this Wikipedia page, https://en.wikipedia.org/wiki/Wounded_Knee_Massacre, making it more likely it was the one you saw. I thought at first you might have conflated several quotes, but I didn't see ones specifically referencing animals and "loneliness."