What...not even 'meaning'!?
I can assure you, Count, that The Oxford English Dictionary claims one meaning of 'milt' - or its variant 'melt' - is (I quote)..."the spawn of fish". People could easily have imagined that it might apply to horses, too.
I assumed from the question that the opening word, 'Ya', was the typical way of pronouncing 'you' when it precedes an insult. I deduced, therefore, that the context was a name-calling one.
In these situations, the opponent's parentage is often called into question...as in "You b@$t@rd!"...or (old-fashioned, I admit) "Spawn of the devil!"...or "You son of a bitch!"
These facts explain my answer. However, as I had never heard of the phrase before, I was well aware that I might be wrong and that was why I used tentative words such as probably, assume and suggestion.
Perhaps all will eventually be revealed.