Legolas senses the Balrog before the rest of the party

Arrows fell among them. One struck Frodo and sprang back. Another pierced Gandalf’s hat and stuck there like a black feather. Frodo looked behind. Beyond the fire he saw swarming black figures: there seemed to be hundreds of orcs. They brandished spears and scimitars which shone red as blood in the firelight. Doom, doom rolled the drum-beats, growing louder and louder, doom, doom.

Legolas turned and set an arrow to the string, though it was a long shot for his small bow. He drew, but his hand fell, and the arrow slipped to the ground. He gave a cry of dismay and fear. Two great trolls appeared; they bore great slabs of stone, and flung them down to serve as gangways over the fire. But it was not the trolls that had filled the Elf with terror. The ranks of the orcs had opened, and they crowded away, as if they themselves were afraid. Something was coming up behind them. What it was could not be seen: it was like a great shadow, in the middle of which was a dark form, of man-shape maybe, yet greater; and a power and terror seemed to be in it and to go before it.

It came to the edge of the fire and the light faded as if a cloud had bent over it. Then with a rush it leaped across the fissure. The flames roared up to greet it, and wreathed about it; and a black smoke swirled in the air. Its streaming mane kindled, and blazed behind it. In its right hand was a blade like a stabbing tongue of fire; in its left it held a whip of many thongs.

‘Ai! ai!’ wailed Legolas. ‘A Balrog! A Balrog is come!’

Gimli stared with wide eyes. ‘Durin’s Bane!’ he cried, and letting his axe fall he covered his face.

‘A Balrog,’ muttered Gandalf. ‘Now I understand.’ He faltered and leaned heavily on his staff. ‘What an evil fortune! And I am already weary.’

The Fellowship of the Ring, Book Two, Chapter 5: The Bridge of Khazad-dûm.