In Ribbontails Series Aman5 there are creatures called Kuutamian animals. These animals are sapient and can have any form. The creator uses the excuse that they have three DNA strands, but how realistic is this? If the three strands could not work, how else can I have each individual animal being different?
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Kuutamian animals are unlikely to have been designed with any particular scientific rigor being applied to the process. Indeed, the link provided is to an art site, and the author identifies this world as 'escapism'.
The main problem with a species so amorphous that they could grow in any imaginable form is that so many of the potential forms have a low probability of survival to be able to reproduce, meaning that any species with a more tightly-regulated body form would have the advantage of having a higher baseline survivability.
The only way I can imagine such a wide variety of body forms is that - unlike terrestrial lifeforms - these organisms have three levels of organisation: Cells, polycellular units, and polycellular unit colonies.
Cells should be familiar to us, as we are made of them.
Polycellular units are effectively small, semi-independent animals which can combine to both specialise in a particular metabolic/physical/mental/sensory process while sharing the specialist processes of other polycellular units, while not becoming incapable of any process other than their specialty. A Polycellular unit may be a few centimetres across. Each would be capable of everything that all the others were capable of, but also capable of specialising.
A Polycellular Unit Colony is simply a group of Polycellular Units - probably, but not necessarily genetically identical. They organise themselves into a form that follows the whim of the being, and as they link, they may share a circulatory system, and form "external" pockets (that are still within the PU Colony) that can be used for processes such as respiration and digestion.
An interesting side effect of a body plan such as this is that the Colony need not be static, and could be re-modelled or completely transformed in a relatively short period, or even split into several parts and then recombined at a later time, effectively giving the being the capability to transform at will. However, it can be expected that if interrupted during such a transformation, its mind would not be particularly well-integrated, and it would suffer from mental deficiencies until it could lock its polycellular units together and learn how to work with their new physical configuration.