Can this card be discarded from hand to ready a hero or does it need to be in play already, and then discard from play? Here is the card's text:

Action: Discard Westfold Horse-Breaker to choose and ready a hero

Thanks.

It has to be in play. From p. 23 of the rule book:

"Actions
Actions are denoted by a bold “Action:” trigger on a card. Actions are always optional, and can be triggered by their controller during any action window in the game sequence. In order to trigger an action on a hero, ally, or attachment card, the card on which the action is printed must be in play, unless the action specifies that it can be triggered from an out of play state."