Rotate AI towards target, on single axis?

Hi, I'm a beginner and I'm using C# for scripting. My problem is that my AI who is chasing me (it's target), is doing so within all the axis's. I only want it to turn horizontally and not be able to look up or down. This because it interferes with my gravity, it makes him fall through the ground.

Relevant code:

         if (Vector3.Distance(target.position, myTransform.position) < minDistance) <---- Checking if he's in range (don't worry, it's the right variables)
     {
         myTransform.rotation = Quaternion.Slerp(myTransform.rotation, Quaternion.LookRotation(target.position - myTransform.position), aiRotationSpeed * Time.deltaTime);   <---- Rotates towards target in all axis's.
         
         if (Vector3.Distance(target.position, myTransform.position) > maxDistance) <---- Checking if he's too close.
             {
                 myTransform.position += myTransform.forward * aiCombatSpeed * Time.deltaTime; <---- Move towards target.
             }
     }
     else  <---- I want him to randomly roam. This part works.
     {
         transform.Rotate(0, randRot * aiRotationSpeed * Time.deltaTime, 0);
         myTransform.position += myTransform.forward * aiMoveSpeed * Time.deltaTime;
     }

My question is: How do I make this piece of code work for just the horizontal axis's?

 myTransform.rotation = Quaternion.Slerp(myTransform.rotation, Quaternion.LookRotation(target.position - myTransform.position), aiRotationSpeed * Time.deltaTime);  

Best regards!