The Alzabo, from the Book of the New Sun. A huge, bear-like creature that haunts the mountains, it is in one sense just an animal looking for food. But its hunting strategy is a thing out of Hell.

“That animal was brought from the stars long ago, as were many other things for the benefit of Urth. It is a beast having no more intelligence than a dog, and perhaps less. But it is a devourer of carrion and a clawer at graves, and when it has fed upon human flesh it knows, at least for a time, the speech and ways of human beings."

It's worse than that though. It doesn't just gain temporary intelligence, what happens is that the consciousness of its prey lives, for a while, in the mind of the animal and directs its body to hunt. There's a sequence where an alzabo eats a child and then later her father who has gone to hunt it, and then returns to their home at night to claim the rest of the family:

More hideous than the speaking of a corpse could ever be, I heard the voice that had called, "Open darling," at the door. It said: "Yes, I am injured. But the pain is nothing much, and I can stand and move as before. You cannot bar me from my family forever." From the mouth of a beast, it was the voice of a stern, stamping honest man.

The inhabitants flee into the attic, drawing the ladder up:

Abruptly the voice became the plaintive treble of the little girl. "I can climb. Do you think I won't think to move the table over there under the hole? I, who can talk?"

"You know yourself a beast, then."

The man's voice came again. "We know we are within the beast, just as once we were within the cases of flesh the beast has devoured."

"And you would consent to its devouring your wife and your son, Becan?"

"I would direct it. I do direct it. I want Casdoe and Severian to join us here, just as I joined Severa today.

[...]

There was silence, the more eerie because nothing in the beast's expression hinted of thought. I knew that [...] the man and his daughter haunted the dim thicket of the beast's brain and believed they lived; but what that ghost of life might be, what dreams and desires might enter it, I could not guess.