the top part
it a door.
The bottom one
is a horse.
The character means a horse behind door, which is breakthrough? It is a little bit confusing.
I think what the ancient Chinese wanted to say is the dynamic moment when the horse rush to breakthrough at the door, not a horse stands quietly behind a door or inside a stable.
Its small seal script is
The door is much longer than the big seal script, which 100% cover the horse inside. We should still think it as a dynamic moment when the horse rush to breakthrough the door.
Here is my art to the meaning I mentioned above.
The horse broke the door and rushed out.
Its clerical script is
which is similar to its clerical script.
Its current standard printing script for simplified character is
The 門 became 门， the 馬 became 马.
Its Pinyin is Chuang3.