My work is having an event tomorrow and they will have horses. i'm severely allergic to them. what's the best way to prepare for this?

Don't go: The best way to prevent a reaction would be to avoid the allergen (in this case horses). If that's not possible, take an antihistamine before you go (preferably a non-sedating one). If you have asthma or breathing problems, take your controller medications as directed, and bring your rescue inhaler (i.e., albuterol) with you. Despite the above, you may still get a reaction. Talk to your allergist.