If the Deer is a male, it will have antlers, be larger than the other Deer and look more upright. If the Deer is a female, it will have no antlers and look outward more. Baby Deer look like a much smaller (And slightly spottier) version of the female Deer.
A Deer will occasionally Drop Fur when killed, which can be crafted into Armor. They will also drop beef when killed.
A Deer will run away from anything bigger than a chicken. Being a passive Mob, they are peaceful creatures, and will never attack. However, when running, they can be very fast sprinters, as well as high jumpers, and will jump around wildly when trying to escape. Deer can jump over wooden fences. This can make Deer difficult to attack or trap.
Like many untamable Mobs in the Mo' Creatures Mod, a Deer will despawn after some time, making any efforts other than killing them useless. However, as previously mentioned, due to the fact they sprint and jump so frantically, they are difficult to kill.
Like most passive Mobs, Deer can be sent into Love Mode when fed Wheat. Though they can be sent into Love Mode, Deer cannot actually breed. A Deer will be forced to come out of Love Mode if it is attacked. Once given Wheat, a Deer will often forever remain in Love Mode, and must be attacked in order to come out of Love Mode.