proto.js

Proto = {
  clone: function(extend) {
    var F = function() {};
    F.prototype = this;
    var o = new F();

    for (var attr in extend) {
      o[attr] = extend[attr]
    }

    return o;
  }
}

Animal = Proto.clone();

Horse = Animal.clone({
  name: "horse",
  speed: 45
})