You may have heard that if you cut an earthworm in half, both halves will grow back into a full worm. That’s an urban legend. (They have amazing regenerative powers, but a severed back end will never be able to grow a new head.) But if you’re wondering how you could bring this into an RPG session, the answer is self-evident: Toss in a couple trolls.
More intriguing is the case of the bulldog ant:
“But the bulldog-ant of Australia affords us the most extraordinary example of its kind; for if it is cut in two, a battle begins between the head and the tail. The head seizes the tail in its teeth, and the tail defends itself bravely by stinging the head: The battle may last for half an hour, until they die or are dragged away by other ants. This contest takes place every time the experiment is tried.” (Arthur Schopenhauer, The World as Will and Representation)
I think the behavior after severing would be easy enough to model as a confusion effect on both halves of the creature. But the real question would be determining when and how the creature would get severed in the first place. A few thoughts:
(1) Some sort of modification on the mechanic used for severing a hydra’s head. The question, however, is how you can motivate a PC to actually make the sunder attempt. (This problem also exists with hydras.)
(2) Treat all weapons as effectively vorpal against the creature. When somebody confirms a crit off of a natural 20, the creature is effectively severed.
(3) Treat it as an effect of bloodying the creature (reducing it to half hit points).
(4) Treat it as an effect of killing the base creature (which is effectively transformed into two new creatures). Maybe there’s only a percentage chance that this happens.
(5) Set a “Break Threshold” for creature: If somebody deals more damage than the break threshold in a single attack, the creature is severed.
Ideas like this work best if they are accompanied by lore in the game. As a player, I enjoy fighting creatures with fantastical strengths and weaknesses. That enjoyment vanishes if the strengths and weaknesses seem random or are totally dissociated from the world I inhabit.
If I, as a GM, were to introduce the concept of ants that fight themselves when split in half, I would do so in two stages. The first stage would be very early. Perhaps I would even go for level 1. On death, or if struck by a slashing weapon doing, say, more than half their HP, the medium-sized fire ants would split and flail around at each other. I would make a big deal out of this effect in the description, but in game effect limit it to, say, 1 attack (with penalties) from each end. I would then later introduce a much scarier version. Perhaps I would even make an explicit comparison. Since the players had already been introduced to the idea that monsters of this type can be split in half, I would be free to make that a much more important part of the battle.