Octocopter makes perfect sense. Everyone understands immediately what it means, and that's the only purpose of language: to convey ideas. It should be clear, which this is, and concise, which this is.
Fidelity to ancient Greek is not, and should not, be a goal for English.
gyrocopter, helicopter, quadcopter, hexacopter, octocopter, parcelcopter, and—most famously—
roflcopter, https://en.wiktionary.org/wiki/roflcopter#/media/File:Roflco...
They all have their own dictionary entries.
Octocopter makes perfect sense. Everyone understands immediately what it means, and that's the only purpose of language: to convey ideas. It should be clear, which this is, and concise, which this is.
Fidelity to ancient Greek is not, and should not, be a goal for English.