Kangaroos are marsupials and deer are placental mammals.
Most species of kangaroo have "embryonic diapause". This means female kangaroos can often be in a state of almost permanent pregnancy, with an embryo "suspended" until the mother's body is ready to release it - whether that be when another joey is sufficiently weaned, or feeding conditions have improved.
Kangaroos and deer move differently. Deer use all four legs to run and leap. Kangaroos mostly just use their hind legs. Deer can run very fast, but cannot sustain this speed for long distances. Kangaroos are able to travel long distances at a high speed, expending very little energy. They are very energy-efficient, and this is linked directly to the physical action of bringing their hind legs up with each hop. Every hop literally refills the lungs. Kangaroos have large, strong tendons in their hind legs which act as "springs". The springing motion requires less energy than running does, so kangaroos are able to bound for longer distances than other mammals with the standard four legs can run. They have strong back legs and elongated hind feet for bounding.
Kangaroos have long, strong tails for balancing. Deer have a small tail which does not serve any significant function.
