What did Ralph find when he hid in the thicket?

Good one. It's right at the very end of the book, and therefore is a moment that isn't much commented on - despite the fact that it's actually (I think, at least) very important for any essay thinking about the symbol of the Lord of the Flies himself - the pig's head on a stick.

Some context first: Ralph is now the only one left who hasn't been killed (Simon, Piggy) or encompassed into Jack's savage tribe. They're hunting the island for him, and haven't yet found him. So he's hiding in the greenery and trying to watch them without being watched. And interestingly, at one point, travelling across the island, he stumbles into Simon's 'place', the covered sheltered leave hideaway where the candle buds grow and where the Lord of the Flies on his stick argued with Simon shortly before Simon's death.

Golding makes it quite clear that it is the same place:

The slanting sticks of sunlight were lost among the branches. At length he came to a clearing in the forest where rock prevented vegetation from growing.

The darkness of Jack and Roger, though, has spread across the island:

Now it was a pool of shadows and Ralph nearly flung himself behind a tree when he saw something standing in the centre; but then he saw that the white face was bone and that the pig's skull grinned at him from the top of a stick. He walked slowly into the middle of the clearing and looked steadily at the skull that gleamed as white as ever the conch had done and seemed to jeer at him cynically. An inquisitive ant was busy in one of the eye sockets but otherwise the thing was lifeless.
Or was it?

Ralph stands face to face with the Lord of the Flies - the symbol (and see elsewhere in my answers to this group for more on this) of all evil and the devil himself. It is - symbolically - man versus evil.

He stood, the skull about on a level with his face, and held up his hair with two hands. The teeth grinned, the empty sockets seemed to hold his gaze masterfully and without effort.
What was it?

Ralph somehow knows that there is something deeper to the meaning of the pig's head on a stick:

The skull regarded Ralph like one who knows all the answers and won't tell.

And then, just like in the novel, Ralph decides to fight evil head on:

A sick fear and rage swept him. Fiercely he hit out at the filthy thing in front of him that bobbed like a toy and came back, still grinning into his face, so that he lashed and cried out in loathing. Then he was licking his bruised knuckles and looking at the bare stick, while the skull lay in two pieces, its grin now six feet across.

It's an ominous moment. What happens when man tries to take on evil forces? Evil gets worse: the grin spreads from one foot to six feet. Trying to destory evil has made it somehow larger and more powerful (both Simon and Piggy would agree with that sentiment).

This deeply symbolic and interpretable little moment - where Ralph finds the Lord of the Flies in the thicket - shows us the key theme of hte novel: the strength and power of evil. The darkness of man's heart spreads.