Ikora mutate game objects
The resulting creature is the same game object that you already had on the battlefield before; all it does is change characteristics. Similar to if someone cast a Turn to Frog on it.

This is the case whether you put the original creature card on top or on bottom of the new mutate card.  

From the Ikoria mechanics article:


  It simply makes the creature that was already on the battlefield change characteristics. If that original creature was tapped, the new merged creature is tapped. If it had any counters on it, or Auras or Equipment attached to it, it still does.


Also in the overview video, it says 


  It’s still the same creature it was; it just changed characteristics.