Can you play an Ambush unit to cancel the Mutating Horror Kha'Zix?

No, playing a unit with Ambush will not cancel Kha'Zix, Mutating Horror's triggered ability if the trigger conditions were already met when he entered the battlefield.

According to established RiftJudge FAQs (e.g., #10198, #10934, #10748), the condition "if an enemy unit is alone here" is evaluated at the exact moment the unit gains the "Attacker" or "Defender" combat designation.

Here is the breakdown of the interaction:

1. Trigger Condition: When you move Kha'Zix, Mutating Horror into a battlefield where the opponent has only one unit, he gains the "Attacker" (or "Defender") designation. At that exact moment, the game checks if an enemy unit is "alone." If the condition is met, the ability is placed on the chain.
2. Reaction: Your opponent can play a unit with Ambush as a [Reaction] to the triggered ability being added to the chain.
3. Resolution (LIFO): The unit played via Ambush enters the battlefield first. Then, Kha'Zix's triggered ability resolves. Because the condition was already satisfied at the moment the unit triggered, the game does not re-evaluate the board state. The ability will resolve successfully, and Kha'Zix will receive his +2 Might bonus and gain 2 XP.

In short, you can use Ambush to add units to the battlefield, but it does not retroactively negate the "alone" check that Kha'Zix performed the moment he entered and designated his attack.