I think what you are looking for is a "distance" expression restriction to the Head motions from the Neck or body of the dragon
You could try adding an extra Condition to your Mouse events
Use something like this
Mouse(distance dragonheadx, dragon heady, mousex, mousey,)
less than or equal to (the distance you want to restrict the movement to)
then your usual events and actions
another way is to "Physics Joint" everything in the Dragon head and neck.. and use a constant physics force that pushes the Dragon head towards the mouse POS but is restrained by the joint force...its a fine balance but it can work good
Obviously alot of experimentation is required.
Did that make sense?
Ive used this technique in another game and it works fine...
Just play around with the factors..