Original: For the movie, Disney took the liberty of changing Bambi's species into a white-tailed deer from his original species of roe deer, since roe deer don't inhabit the United States, and the white-tailed deer is more familiar to Americans.
Simplified: For the movie, Disney changed Bambi's species into a white-tailed deer from his original species of Roe Deer, since roe deer don't live in the United States, and the white-tailed deer is more familiar to Americans.