A thought experiment I sometimes dabble in is to take a basic creature design and create a whole bunch of other critters evolved from it. Not merely different breeds but lifeforms that live in entirely different areas and fill completely separate niches; the equivelant of crafting "reptiles" and not just "lizards".
Alien Evolutions
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Crescent Critters -- base form has 2 limbs, 4 eyes, and a crescent-shaped body. The blue one with the "legwamers" is the most basic.
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Pentalegs -- base form has round body, 5 limbs, antennae, and is radially symmetric. The little pink star-patterned one is the most basic.
And if you were wondering how that flying eyeball would've evolved:
