I'm sorry to hear that Marsala is having an issue this evening. This could be a number of things from a puncture wound (stick, nail, wire, ect) leading to the small wound and drainage you are noting. It could even be a bite of some sort (spider, wasp, snake, ect). Could even be an abscess that has been developing and finally opened up and is draining.
I would recommend taking her temperature (normal is 98.5 to 100.8 F), anything over 101.0F is considered a fever. This would be suggestive of infection or significant inflammation. Also, monitor feed/water intake. As their body temperature rises, I see a number of horses begin to go off feed at about 102.0F
This might not be an emergency per se, but I would recommend that your veterinarian see her tomorrow if possible.
Regardless of the cause, your veterinarian will most likely want to place her on antibiotics. Generally I start with Trimeth-sulfa (SMZ) 800mg/160mg tablets at 1 tablet per 100 pounds twice daily as it is fairly broad spectrum and develops good levels in the skin.
If this is an abscess, your veterinarian may want to take a culture sample to determine the bacterial organism causing it and what antibiotics it is sensitive to. One very common one is Corynebacterium pseudotuberculosis (pigeon fever) and may only respond to specific antibiotics. If it is an abscess it may need to be opened up to facilitate further drainage, and lavaged to kill organisms in the tissue.
Things you can do while waiting for your veterinarian to see her is cold hose the area for 10-15 minutes 2x per day, wipe away any debris, and use a triple antibiotic ointment such as neosporin topically to prevent/minimize any other organisms from creating further issues.
Depending on her temperature, you may need to use bute (based on your veterinarians recommendation) to reduce any fever and soreness associated with the wound.
Also, watch for any other lumps/bumps/swellings that may appear lower. As the inflammation increases and swelling persists, gravity may begin to pull that fluid further down the chest or even down the front limbs.
I hope this has fully answered your questions. If not, or if you have any further questions, please don't hesitate to ask.