Read the linked post. The main reasons we can't define words however we like are because that leads to not cutting reality at the joints and because humans are bad at avoiding hidden inferences. Not being a biologist, I can't assign an ideal definition to "duck", but I do know that calling lobsters ducks is clearly unhelpful to reasoning. For a more realistic example, note the way Reactionaries (Michael Anissimov, Mencius Moldbug and such) use "demotist" to associate things that are clearly not similar.