Durin's Bane

What the text says

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.

The Fellowship of the Ring, ‘The Bridge of Khazad-dûm’

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.

The Fellowship of the Ring, ‘The Bridge of Khazad-dûm’

His enemy halted again, facing him, and the shadow about it reached out like two vast wings. It raised the whip, and the thongs whined and cracked. Fire came from its nostrils.

The Fellowship of the Ring, ‘The Bridge of Khazad-dûm’

The fire in it seemed to die, but the darkness grew. It stepped forward slowly on to the bridge, and suddenly it drew itself up to a great height, and its wings were spread from wall to wall; but still Gandalf could be seen, glimmering in the gloom; he seemed small, and altogether alone: grey and bent, like a wizened tree before the onset of a storm.

The Fellowship of the Ring, ‘The Bridge of Khazad-dûm’

It was both a shadow and a flame, strong and terrible.

The Fellowship of the Ring, ‘The Mirror of Galadriel’

Notes

Tolkien never gives the Balrog a clear outline. It first comes as a great shadow with a dark form inside it, "of man-shape maybe, yet greater". On the bridge the shadow about it "reached out like two vast wings", and a few lines later "its wings were spread from wall to wall". Aragorn remembers it afterwards as "both a shadow and a flame". The text gives it a streaming mane that kindles, a blade like a tongue of fire, a whip of many thongs and fire from its nostrils. It never mentions horns.