"At the time of King Henry II when Bartholomew de Glanville kept the castle at Orford, it happened one day that some fishermen who were fishing, caught a wild man in their net. They handed him over to Bartholomew to admire. The wild man was completely nude, and appeared as a human in all his parts. He had hair on his head, but ruined and wrecked in appearance. He had a really long beard, and he was really too hairy and shaggy on his chest. Bartholomew had him guarded for a time, day and night, so that he could not go back to the sea. Whatever was brought to him, he ate avidly. Indeed he ate fishes raw rather than cooked, but he would squeeze the raw fish between his hands and suck the juice out and thus he would eat. He emitted not one word either because he did not want to or he could not, even when he was suspended by his feet and subjected to frequent dire tortures. Although he was brought to church, he showed no signs of veneration or any sign of belief, by either bending the knee or inclining the head with which one could discern something holy. He always eagerly sought his bedchamber when the sun went down and would lie there until the sun came up.
It happened one day, that they brought him to the sea and set him in the sea, held on to by a net, three lines thick. Who, seeking the deepest part of the sea, passing through the net, was emerging again and again from the bottom of the sea and was looking at those watching from the shore of the sea a long while. Often he dipped down and blow out a bit of water as if insulting those watching because he escaped their net. And when he played in the sea thus a while and now all hope was gone that they would recapture him, suddenly he came, swimming in the waves, again all the way to them. And he remained with them again for two months. But when those keeping him became negligent and showed distaste for him, he fled in secret to the sea and never appeared afterwards. If however he existed as a mortal man, presenting himself as some human type of fish, as if an evil spirit was hiding in the body of the submerged man, just as may be read in the life of St. Audoeni, one cannot easily categorize him. Most of all because so many wonders are being told by so many about these events."
That was a quick and dirty translation on my part. The story seems part Gollum, part Free Willy. I couldn't say for sure that Tolkien had read Chronicon Anglicanum (it is not a particularly obscure text) and that this incident inspired his portrayal of Gollum (Gollum was not this hairy) but it struck me as very similar.