Checks to see if the player is mounted or not.
1 if the player is currently mounted, nil otherwise
mounted = IsMounted()
If the player is mounted then the value of mounted will be 1. However if the player is not mounted then mounted will have a value of nil. To save space, instead of assiging the return value of IsMounted() to mounted, IsMounted() can be inserted into the code like a number.
/script if IsMounted() == nil then...
In this case if the player is not mounted then the code after then will be excuted.
Unknown which patch IsMounted() was added in.