The idea is hardly new. Olaf Stapledon has at least one collective species who evolved from bird-like flocking creatures in Star Maker, while a there is a Poul Anderson story (sadly, I cannot recall which one) in which intelligences are formed of three members distinct symbiotic species linking physically. And there are always the Borg.
I'd like to pose something that is individually large enough for a single body to have intelligence (perhaps at the level of a chimpanzee) and a member of a small enough collective (perhaps half a dozen members) that it cannot be looked at as disposable the way an intelligent ant colony might think of a single ant. Currently I find myself picturing it as small (dog sized) and with a body plan like a compact centaur. In fact, one my sources for this idea is a discussion the possible uses of Baccus 6mm Centaur infantry as 25mm figures mounted 3-4 to a base and claiming that they used the group mind to concentrate the fire of individual small lasers.
I expect something like this might evolve from pack hunters who used the ability to coordinate hunting. In the ecosystem where this evolved I would expect it to be a widespread capability, like magnetic orientation in many species of bird. Special pleading would be required to explain how a "bioradio" of sufficient bandwidth to distribute cognitive processes is present in this ecosystem; however it is not hard to see how it could confer useful advantages once evolved.
The interesting questions are social. There are a number of dimensions on which "micro-centaurs" could organize themselves socially. Lets express them as diametric opposites. Let us also assume that the individual units are either male or female, and that there is some level of sexual dimorphism and a distinction of capabilities (although ranging widely between individuals) roughly aligning with 20th century human gender stereotypes -- because, after all, humans are all we have available to play head games with:
- Genderist vs Multi-Genderists: Genderists believe that a single collective should all be one one gender, while multigenderists believe that an collective can be made of any combination of unit genders. I can see genderism being considered natural where sexual division of labor is commonplace such as hunter-gatherers while more settled societies may want to mix capabilities within a single individual.
- Immortalists vs Discretest. Immortalists believe that the collective should take advantage of its potential immortality by socializing offspring as part of the collective with older members; new collectives would be formed by older members separating along with some younger members. While units would die, the collective would like on. Discretests believe that each collective should experience the full cycle of life and death. Offspring are socialized within age groups and raised as distinct individuals, dying when all units have passed on.
- Extentionists vs Independentists. Extensionists believe in forming super-collectives to work together on serious problems or simply for the self-transcendent experience. Independentists believe that the right to self-determination of the individual collective is foremost.
And this does not get at questions like reproductive and child rearing practices. Do multi-genderists reproduce with themselves, or are there the equivalent of incest taboos? How do inheritance laws work? How do class structures work within each society? Are domestic animals somehow integrated in the collective in an evolutionary path like the one that led to dog/human symbiosis or would that evolution stop at a lower level of integration?
And what is a collective like to interact with? Do they use voice as well as bioradio to communicate with each other? How about with other collectives? If they can speak, if only to talk with humans, would there be one spokesbeing or might conversation rotate around the members like a well-timed comedy routine? Would this all vary by the individual (a word that rapidly loses its utility)?
What would an isolated unit be like? Sure,
Heck, if they speak, what sort of pronouns might they have for multi-genderists? Would they have a family of pronouns that expressed two-thirds-female-one-third-male? Different pronouns for units and collectives? Genderist cultures would probably have different pronouns from muli-genderist, and would have to use borrow-words from one-another's languages to hold a conversation.
Add to all this the fact that it "takes all sorts to make a world" and the mini-centaurs would have their own managers and workers, criminals and heroes, artists and engineers and I think you could have a very interesting society to explore.