Ya mares melt?

[Baby02]

What exactly does this mean?

[Quizmonster]

Probably much the same as 'you son of a bitch'. One meaning of 'melt' - a variant of 'milt' - is the roe/semen of a male fish. One can only assume that the association with a female animal - a mare - is intended as an added insult! Whatever the intent, the overall suggestion is, I guess, that the person addressed is the offspring of an animal.

[Count_Emmup]

In what context have you seen it, Baby?

Because I don't believe a word of the above.

[Quizmonster]

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.

[Quizmonster]

For you, Baby02...Here's another possibility..
'Mare' in the sense of 'nightmare' was the name given to a goblin, male or female, believed to crouch on one's chest or even have sexual relations with one during sleep, thus possibly causing unpleasant dreams. It is very ancient and found also in the French word for 'nightmare' which is 'cauchemar'.
It's possible, therefore, that your phrase has nothing to do with horses but means "You spawn of an evil goblin!"

[HWKE]

I was raised in Lancashire and 'Pig's Melt' was offal - specifically brawn.
To be referred to as pig's melt was an insult implying you were the useless parts of an animal.
I would think that 'Mare's Melt' is much the same but from a different part of the country.

[Quizmonster]

Ah, so it seems from <b<HWKE's answer that we probably are dealing with a name-calling context and that it is common in some areas to use the idea of an animal's organs as an insult. That's basically what my first answer suggested, too, though I'd never heard any such thing.