Birds. Or fish, depending on how you set it. A crowd of particles moving as one body, splitting around obstacles and rejoining, with nothing coordinating them.
Every particle follows just three rules, and only ever looks at the handful of neighbours near it. Do not crowd anyone. Head roughly the way the neighbours are heading. Drift toward where the neighbours are. The three are weighted and added together into a nudge to its course.
That is the entire program. No particle knows there is a flock, nobody is leading, and there is no plan. The coherent motion of the whole group is a side effect of everyone attending to their immediate neighbours.
These are Reynolds boids, from 1986, and they are the standard example of emergence: complicated group behaviour arising from simple individual rules and nothing else.