The gribble was a hybrid creature composed of a Tribble and a Gerbil, created by Dr Bethom as an experiment to induce human-like intelligence in non sentient creatures. Unlike the other experiments of Dr Bethom, including Janos, the Gribble did not suffer from an intelligence decay and was in fact smarter then the doctor himself.
The Gribble used it's abilities to manipulate both Dr Bethom and the scientists of the Daystrom Institute on Alpha Sigma IX to work for it in state of the art labs, developing weapons and technologies which were sold to "shady" parties.
It was found out by the collective efforts of both Jean-Luc Picard and Mackenzie Calhoun when Calhoun sought out help to solve the intelligence de-generation of Ensign Janos, the Gribble offered to save Janos in exchange for it's survival but was eaten by Janos shortly afterwards. (NF novel: Stone and Anvil)