I was replaying Super Mario Bros. 3 again recently and was trying to get all of the White Mushroom Houses to spawn. You need to collect a certain number of coins in certain levels to make one appear. I was curious how they were actually implemented in the game, so I had a little look. It's fairly st...
TL;DR:
You need to collect a certain number of coins in certain levels to make one appear (...), but there are a couple fun things about it
I think there's more to the coin ship, that unfortunately isn't covered in the article. I know for a fact that the hammer bro on the World 1 map turns into a coin ship if you finish 1-1 with 55 coins AND end the level with time remaining being an odd number.