Is a nine-tentacled octopus . . .
. . . an impossible creature? Only if the name "octopus" expresses an essence, I think. What, then, would the creature in this photo above be called? A "nonopus"? But such names do not express essences. The 'nonopus' above looks to be an octopus that lost a tentacle, from the stump of which sprang two new tentacles.
If, therefore, like the Queen in Through the Looking Glass, one could believe "as many as six impossible things before breakfast," a nine-tentacled octopus would not be one of them.
A nine-sided octogon would qualify, however, for the name expresses an essence. Necessarily, an octogon has precisely eight sides. A nine-sided octogon would therefore be impossible. Knowing this, however, could anyone believe in such an absurdity?