There is froth exuding from the nares.  This is an indication of terminal pulmonary edema.  The edema fluid has lots of protein in it and as the horse takes its last few gasps, that proteinaceous fluid gets whipped into a froth that comes all the way up the trachea and out the nares.
Froth extruding from nares
Well, looky here.  This is the reason for the froth at the nares.  This poor horse had lungs that were absolutely loaded with fluid.  Note the extensive accumulation of edematous fluid within the interlobular septa and at the ventral margin.   There is also hydrothorax.
Pulmonary edema
Wow, hydropericardium.  There is WAY too much fluid within the pericardial sac here.
Hydropericardium
A close look at the papillary muscle of the myocardium reveals a focal area of hemorrhage - this is very acute myocardial hemorrhage and necrosis.
Acute myocardial hemorrhage and necrosis