// with field
private int sheepCounter;

// using in prepare method and anywhere else you create Sheep objects
Sheep sheep = createNewSheep();
// (or just replacing "new Sheep()" with "createNewSheep()" everywhere)

// to call this method
private Sheep createNewSheep()
{
    return new Sheep(sheepCounter++);
}

// add  field
private int identificationNumber;

// change constructor to (or add)
public Sheep(int idNum)
{
    identificationNumber = idNum;
}

// add method to return id
public int getID()
{
    return identificationNumber;
}

"I'm sheep number " + this.toString()

private int getActorCount()
{
    for(int i = 0; i < getWorld().getObjects(<Actor>.class).size();i++)
    { 
        if(this.toString().equals(getWorld().getObjects(<Actor>.class).get(i).toString()))
             return i+1;
    }
    return 0;
}

int ident = this.getWorld().getObjects(this.getClass()).indexOf(this)+1;

int ident = getWorld().getObjects(getClass()).indexOf(this)+1;