Ecological Group Think

Animals work in packs and herds. This could work by having one mob in a spawn group marked as the alpha, and the rest as betas. It's code should be identical to the mob species except for this: mobs in the area defer their “choices” to the alpha, and the alpha is aware of what happens to the betas and will act accordingly. The alpha will act as if what has happened to the betas has actually happened to it, and all the betas will follow suit. This seems like an elegant way to add herd/pack mentality.

Also the betas would attempt to stay within some 20 blocks or so of the alpha.