Edit in JSFiddle

Function.prototype.extends = function (Parent) {
    var F = function () {};
    F.prototype = Parent.prototype;
    this.prototype = new F();
    this.prototype.constructor = this;
    this.superclass = Parent.prototype;
    return Parent;
}

function extend(Child, Parent) {
    var F = function () {};
    F.prototype = Parent.prototype;
    Child.prototype = new F();
    Child.prototype.constructor = Child;
    Child.superclass = Parent.prototype;
}

function Animal(title) {
    var tree = [], level = 0;
    this.kingdom = this.family = arguments.callee.name;
    this.species = this.constructor.name;
    this.voice = '';
    this.title = title;
    this.greet = function (){
        return this.kingdom + ' ' + this.title +
                    ' is an ' + this.species.toLowerCase() + (this.voice?
                    ' and it`s says ' + this.voice + '!':'');
    };
    this.outLegs = function () {
        var msg = '';
        if(this.legs){
            msg = this.family + 's have ' + this.legs + ' legs';
        } else if(this.family) {
            msg = this.family + 's do not have a legs';
        }
        return msg;
    };
    this.getTree = function(){
        return tree;
    };
    this.getLevel = function(){
        return level + ') ' + this.species;
    };

    // Closure
    this.init = (function(){
        this.init && this.init();
        return function(){
            tree.push(this.family);
            level++;
        }
    }());
    this.init();
}

extend(Cat,Animal);
function Cat() {
    this.family = arguments.callee.name;
    this.voice = 'mieu';
    this.legs = 4;
    Cat.superclass.constructor.apply(this,arguments);
    this.init();
}

Snake.extends(Animal);
function Snake() {
    this.family = arguments.callee.name;
    this.voice = 'ssss';
    Cat.superclass.constructor.apply(this,arguments);
    this.init(); // How to call / Is it possible "private" or parent class function?
}
Snake.prototype.init = function() {
    // Polymorphism ?
    return Animal.init.call(this) + ' ...Sh-sh-sh';
}

Dog.prototype = new Animal(); // Why?
function Dog(){
    this.family = arguments.callee.name;
    this.voice = 'woof';
    this.legs = 4;
    Animal.apply(this,arguments);
    this.init();
}

function Beagle(){
    this.family = arguments.callee.name;
    this.prototype = new Dog();
    this.legs = 4;
    this.color = 'White-brown';
    Dog.apply(this,arguments);
    this.init();
}

function Boxer(){
    this.family = arguments.callee.name;
    this.legs = 4;
    this.color = 'Tiger';
    // Without prototype!
    Dog.apply(this,arguments);
    this.init();
}

var pets = {
    'Creature': Animal,
    'Puh': Cat,
    'Sharik': Dog,
    'Kaa': Snake,
    'Lex': Cat,
    'Woo': Snake,
    'Gog': Beagle,
    'Rex': Boxer
};

for(pet in pets){
    var myPet = new pets[pet](pet);
    console.log(myPet.getLevel(), '(' + myPet.getTree().join(' > ') + ')');
    console.log(myPet.greet());
    console.log(myPet.outLegs());
}

//console.log((new Dog()), (new Boxer()));
//console.log((new Animal()), (new Cat()), (new Boxer()), (new Beagle()));
//console.log(Dog.prototype, Cat.prototype, Boxer.prototype, Beagle.prototype);

div {
    outline: red solid 1px;
    width: 100px;
    height: 100px;
}