Kannik
Legend
The PHB states that a horse, when being ridden, uses its action to move, and thus only you, the person riding the horse, get an attack for the round.
A co-player wants to know if the horse stands still if it can attack. While a centaur can do so, I'm guessing having a horse rear up and attack (somewhat unawares the rider would be as to when it would do so) would prevent the rider from getting much of an effective attack off, so it would be either the rider getting a full action or the horse getting an attack. Or, at best, they'd both get a single attack...
Is there any official ruling on this? }
Kannik
A co-player wants to know if the horse stands still if it can attack. While a centaur can do so, I'm guessing having a horse rear up and attack (somewhat unawares the rider would be as to when it would do so) would prevent the rider from getting much of an effective attack off, so it would be either the rider getting a full action or the horse getting an attack. Or, at best, they'd both get a single attack...
Is there any official ruling on this? }

Kannik