The Loch Ness Monster’s Song by Edwin Morgan.

You can hear Edwin Morgan read the poem himself here.

A note from the Museum of Hoaxes:

According to a Rice University webpage, in 1991 the poem was reprinted in 100 Poems on the Underground, and had this explanation appended to it:

“The author explained in conversation that the lonely monster rises from the loch and looks round for the companions of his youth — prehistoric reptiles — and, finding nobody he knows, he descends again to the depths after a brief swearing session. This was confirmed by a nine-year-old boy in a workshop, who said the monster was ‘looking for a diplodocus’. When asked how he knew that, he said, ‘It says so.’ It does.”

Sure enough, if you read the poem closely, you can tell that the monster is looking for a diplodocus, and does then start swearing.