How are you meant to do this?

[Iggleboz]

 

[DevAwsome1]

 

[Sweetenerera]

it's lucky based

 

[Rawrbin]

The one next to it is called "Hatch a pig", I've already unlocked this one.
There's a small (REALLY SMALL), chance of a pig coming out of an egg, instead of a chicken.
I believe there's also a REALLY small chance for an iron golem to spawn instead of a snow golem.
Not sure about this, but the pig one works like this, so I suppose so does the Iron Man.

 

[matcha]

Pretty much what everyone else above already said, though this might be worth a read if you're interested in the other achievements.