Kangaroos: I think they are the ones who can't walk (by which I assume the question to actually mean 'move under their own power') backwards, because they are designed to hop, and they can only do that forewards. However, so as not to agree with Bonj completely, which, obviously, I try not to do, on principle
, to say that their legs are horizontal is wrong. Only part of their leg is generally in a horizontal plane, and that is in fact their foot (metatarsals and phalanges). The rest is in a more upright position.
In responce to the idea that they only have two legs, to the other animals' four, no, they have 4 limbs like any other, they just don't generally put them to the ground. 'Arms' and 'legs' are a rather anthropoid concept.
And no, Arbitrary, elephants do have legs, and yes they do have knees, but only two, so don't start that elephants have 4 knees thing....