Sinking/Shadow Cows

I was flying around in Creative Mode and looked down. I thought they were black sheep at first but as I fell from the sky, it turned out it was cows! two were shadowed in the daytime. One roamed off and the other stayed shadowed and sank right into the grass. There werent even any trees around as I was in the Plains. I had to break two grass hill blocks the cow was in then the shadowing stopped at that and the cow I freed went away like nothing happened!