Please give more information. What would the deer look like? What would it do? What would you get when you kill it?
Also, for future reference, please make the title less vague. The title of this thread should be something like, "New Mob: Deer" instead of "a new animal."
I make suggestions...
→ Ocean Improvements (Part 1, 2, 3)
→ Improved Player-Villager Interactions
→ Balance The Hunger System