I am deeply sorry to break the news, but legendaries (such as Mewtwo) do not produce eggs*. But almost all other pokemon (with genders) can breed with ditto**. Ditto is available in FireRed, LeafGreen, and Emerald. In FireRed and LeafGreen, ditto can be found in the pokemon mansion on Cinnabar Island, Cerulean cave, or route 12.
Note:
*Although you can not breed mewtwo, if you have Emerald version, you can clone it using the Battle Tower method.
**Baby pokemon can not breed with ditto, and neither can another ditto.
Note:
*Although you can not breed mewtwo, if you have Emerald version, you can clone it using the Battle Tower method.
**Baby pokemon can not breed with ditto, and neither can another ditto.