This is a movie from the 1980s, specifically the year 1988.
The monster can indeed accurately be described as "half-woman half-horse" and "very white."
The movie is named after the Unnamable.
Back in the 1800's a lady gives birth to a monster. They decide that
the baby is too ugly to name, therefore the monster is known as the
"Unnamable". The creature brutally slaughters his family, and gets
trapped in a vault. Go ahead to 1998, and some college students have
heard the story about the unnamable and want to check out the vault....
This is based on the HP Lovecraft story of the same name (or lack thereof):
I was not so seriously hurt, but was covered with welts and contusions
of the most bewildering character, including the print of a split
hoof. It was plain that Manton knew more than I, but he told nothing
to the puzzled and interested physicians till he had learned what our
injuries were. Then he said we were the victims of a vicious
bull—though the animal was a difficult thing to place and account for.
After the doctors and nurses had left, I whispered an awestruck
“Good God, Manton, but what was it? Those scars—was it like that?”
And I was too dazed to exult when he whispered back a thing I had half expected—
“No—it wasn’t that way at all. It was everywhere—a gelatin—a slime—yet it had shapes, a thousand shapes of horror beyond all
memory. There were eyes—and a blemish. It was the pit—the
maelstrom—the ultimate abomination. Carter, it was the unnamable!”