Legolas can understand how horses speak

It is likely enough,’ said Aragorn; ‘yet I am not sure. I am thinking of the horses. You said last night, Gimli, that they were scared away. But I did not think so. Did you hear them, Legolas? Did they sound to you like beasts in terror?’

‘No,’ said Legolas. ‘I heard them clearly. But for the darkness and our own fear I should have guessed that they were beasts wild with some sudden gladness. They spoke as horses will when they meet a friend that they have long missed.’

‘So I thought,’ said Aragorn; ‘but I cannot read the riddle, unless they return. Come! The light is growing fast. Let us look first and guess later! We should begin here, near to our own camping-ground, searching carefully all about, and working up the slope towards the forest. To find the hobbits is our errand, whatever we may think of our visitor in the night. If they escaped by some chance, then they must have hidden in the trees, or they would have been seen. If we find nothing between here and the eaves of the wood, then we will make a last search upon the battle-field and among the ashes. But there is little hope there: the horsemen of Rohan did their work too well’

The Two Towers, Book Three, Chapter 5: The White Rider.