Lenia
Every cell here holds a shade instead of a switch, and time moves in slivers instead of ticks. Drop a creature in and it swims off on its own; push the rule a little and watch it hold its shape, spread across the field, or come apart.
The pen lays a rough patch about one neighbourhood across, which is what starts life here. Whether it lasts is the rule's business: on Coral or Labyrinth almost anything you draw takes root, and on Orbium almost nothing does, because the only thing that setting keeps alive is the creature it was found with.
The shape of the rule
On the left, how much a cell cares about its neighbours at each distance out to the edge of the ring. On the right, how fast it rises or falls once it has read them. Everything on this page comes out of those two curves.
Splitting the ring into two or three bands, each with its own weight, is what most of the larger creatures are built on. One band on its own is the setting the swimmers were found at.
A bigger field costs more per step and gives creatures somewhere to go. Changing it starts the field again, because a creature is a fixed number of cells across and there is no honest way to carry it over.