You never know what you'll encounter when you trailer out, and you shouldn't go out unprepared. Here's what we've found to be a practical first aid kit to carry in our horse trailers (in addition to the standard "automobile" first aid kit in the pickup). It all fits nicely in a standard tackle box.
* Garden hose pieces can be used to keep nasal airways open in the event of snake bite or anaphylactic shock. Other items in the trailer should include an extra halter and lead rope, clippers or grooming scissors, an old bath towel, twitch, sponge, and a clean bucket. Be sure to inspect your first aid supplies periodically, particularly at the beginning and end of each "trailering" season!