The Feghoot series is by Reginald Bretnor writing under the pseudonym of Grendel Briarton.

In 2927, Ferdinand Feghoot rescued Vaila, a minor planet in the Hebridean System, from a plague of rats who had left a doomed Cassiopeian freighter. On Vaila, no cat could live, so nothing threatened them. “What can we do, sir?” asked the Laird.

“You can make robot cats,” Feghoot answered. He designed them, and the natives began turning them out. They killed rat after rat; and the sight of them, in their plastic, striped-tabby skins, cheered everyone up. Success was in sight, and a great celebration was being prepared, when word came of a frightening mutation — a pair of huge rats who were devouring the cats,

“They will reproduce!” moaned the Laird. “We are lost!”

“Not at all,” Feghoot said; and he built one last cat. It had no sleek plastic coat. Instead, it was covered with a poor grade of enamel, like an old chamber-pot. Almost at once, a mutant rat saw it and caught it. It was tough, but the rat chewed and chewed. Finally it swallowed-and huge, jagged fragments of the shoddy enamel came loose in its stomach. Soon it died in great agony.

“Wonderful!” cried the overjoyed Laird. “But what happened?”

“He strained at a cat, and swallowed enamel,” said Ferdinand Feghoot. (Ascribed to Edward Truscoe)

(Copyright © 1960 by Mercury Press. First published in THE MAGAZINE OF FANTASY & SCIENCE FICTION, December 1960.)