What mythological creatures could be realistically created? In my world, there are people capable of advanced genetic creation and manipulation. I was just wondering what creatures would be plausible that they could create. For example, a phoenix is a lot less plausible than creating a Griffin or centaur.
None of the ones you mentioned are feasible I would think. Chopping creatures in half and reassembling them as hybrids will not work. Nor would things like defying physics by having 30 foot dragons, or breathing fire for that matter.
Multiheaded snakes are a possibility, giant creatures (including humans) and things like that would be possible. Cyclops, unicorns and suchlike that are more modifications of a form rather than have magical or unrealistic powers.
Maybe focus on the mythical creatures that are actually more based on reality. A lot of creatures were actually inspired by other real animals. The unicorn for example was likely inspired by an extinct Siberian wholly rhinoceros called an Elasmotherium. In fact, many existing creatures were once thought to be entirely mythical such as the gorilla, squid, narwhal, komodo dragon, and platypus.
Basically drop anything half human, humanoid, or chimera-esk and then drop any magical powers that can't be explained scientifically (like maybe breathing fire was actually spitting acidic venom, still leaves burn like scarring and feels like burning). In the end its still just up to you, but I would say try and come up with your handwavium for a bunch of creatures you like and then choose the most believable ones.