things that do not exist; fur on turtles, horns on rabbits
A yojijukugo idiomatic expression used to refer to something that is completely nonexistent or impossible, literally 'fur on turtles and horns on rabbits'. Often used in philosophical or rhetorical contexts.
完全な平等社会など亀毛兎角に過ぎない。
A perfectly equal society is nothing more than a fantasy.
The ideal world he talks about is something that doesn't exist.