In the Hennebique system of construction the bars are flattened at the end and split to form a “fish tail.”
His nostrils filled with the oily stench, and when he opened his mouth to breathe, he closed it again on a fish tail.
He climbed right up on the big green lily-pad, and reaching out, grabbed the end of the fish tail in his beak-like mouth.
But Great-grandfather Frog had much to learn in those day, and so he tried to swallow that fish tail first instead of head first.
To swing a car, motorcycle, etc, from side to side at the rear: causing his rear wheels to spin or the rear end to fishtail, that is swing back and forth (1927+)