According to this principle, every 4th attack will hit the target, while according to my idea, 4 out of 5 attacks will hit the target (although if there is a connection with the Horn or the rear, it may turn out to be only every 2nd attack, or even less). I'll have to test it.
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I tested it - on one side, I placed 1 horse and hid a golem behind it, and on the other side, I placed 6 horses around the golem. With 50 casts, both had a defense trigger rate of 10 (which is what it should be on average). With 100 casts, the trigger rate was 17 and 19 (it should be 20). I didn't run it any further, but one thing is clear - only one unicorn triggers (20%), regardless of their number. This means that the game uses a simple arithmetic algorithm for calculation (they didn't bother calculating probabilities from each subsequent factor, but simply took the probability from their sum).