How do you get heatron to have an egg Pokemon diamond?

it cant because it doesn't have a gender, and it doesn't have a gender because most legendry's don't yave genders

that guy is wrong heatram can be either boy or girl just save befor batling it then if its a girl catch it if its a boy turn the game of then back on till heatram is a girl and this has been tested so when you catch it try using a steelix to mate with haetran