I do not think it is a kangeroo. They "box" each other, and if you are boxing and cannot reverse, you are in big trouble.
I think it is the brown bear that cannot walk backwards. (Due to its weight, low centre of gravity etc, but I cannot explain it scientificologically.)
Elephants should be able to go backwards, otherwise if they take a drink from a river or lake, how would they move, if not backwards.
Polecats, I think, are very agile. So walking backwards is a piece of cake (as it were) for them. Perhaps it is a trick question. None of them walk, do they preambulate?
