Roger is a Hunter ranger that has chosen the Horde Breaker option for the Hunter’s Prey class feature (Player’s Handbook, p. 93):
Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.
Roger fires an arrow at Michael, a creature with the Mounted Combatant feat who is currently astride a horse. Roger then uses the Horde Breaker feature to fire a second arrow at Michael’s mount. Michael uses the second bullet point of the Mounted Combatant feat (Player’s Handbook, p. 168) to change the target of Roger’s attack:
You can force an attack targeted at your mount to target you instead.
Does Michael get targeted by two attacks? Or is the second attack no longer permitted, because Michael is not a valid target for Roger’s Horde Breaker attack?