The concept is that when you use your base class reference to refer child class instance polymorphically, you can call only those methods defined in the base class using the base class reference.
Though the base class reference is holding a child class instance it's not possible and allowed to call child class specific methods.
See below program, a football player is a person and so is a tennis player. you cannot ask a football player to play tennis and vice versa.
Here is the reference type is Object and hence you can only access members of object and not horse.
If you want to use a access a method/property of Horse, you should simple use the horse reference type itself.
Look at it this way, the only reason Java allows you have to have a base class refer to a sub class type is to make use of polymorphism. Conceptually, a base class reference is used to access a property that is common to all subclasses and not a property pertaining to a particular sub class(here name belongs only to horse).
Common properties of several classes are identified and are put in a common class called the Base class. That is the idea behind inheritance.