From the final section of The Tale of Pigling Bland - by Beatrix Potter:
The grocer flicked his whip-- "Papers? Pig license?" Pigling fumbled in all his pockets, and handed up the papers. The grocer read them, but still seemed dissatisfied.
"This here pig is a young lady; is her name Alexander?" Pig-wig opened her mouth and shut it again; Pigling coughed asthmatically.
The grocer ran his finger down the advertisement column of his newspaper -"Lost, stolen or strayed, 10s. reward;" he looked suspiciously at Pig-wig. Then he stood up in the trap, and whistled for the ploughman.
"You wait here while I drive on and speak to him," said the grocer, gathering up the reins. He knew that pigs are slippery; but surely, such a very lame pig could never run!
"Not yet, Pig-wig, he will look back." The grocer did so; he saw the two pigs stock-still in the middle of the road. Then he looked over at his horse's heels; it was lame also; the stone took some time to knock out, after he got to the ploughman.
"Now, Pig-wig, now!" said Pigling Bland.
Never did any pigs run as these pigs ran! They raced and squealed and pelted down the long white hill towards the bridge. Little fat Pig-wig's petticoats fluttered, and her feet went pitter, patter, pitter, as she bounded and jumped.
They ran, and they ran, and they ran down the hill, and across a short cut on level green turf at the bottom, between pebble beds and rushes.
They came to the river, they came to the bridge -
they crossed it hand in hand -
then over the hills and far away
she danced with Pigling Bland!
"Far more important is the Consolation of the Happy Ending. Almost I would venture to assert that all complete fairy stories must have it.(...) I will call it Eucatastrophe (...) the good catastrophe, the sudden joyous end. (...)
"In such stories, when the sudden turn comes, we get a piercing glimpse of joy, and heart's desire, that for a moment passes outside the frame, rends indeed the very web of story, and lets a gleam come through."
From Tree and Leaf, by JRR Tolkien
I would add that the eucatastrophic effect here comes from a prose tale breaking into verse for the final sentence only.