The National Institute of Mental Health (or NIMH) was a health organization operating on Earth since the late 20th century. Throughout the late 20th and early 21st centuries, NIMH experimented on rats and mice by genetically engineering them with human-like qualities.
Unlike Starfleet Medical's experiments with pandas, which were much more pleasant and accomodating, the NIMH experiments were performed in harsh conditions.
One day, the rodents became intelligent and broke out of the lab. They hid from humanity for over a century before in 2156, the rodents stole an United Earth NX-class starship, the Currie, out of drydock. The rodents colonized the planet Ratnia, which was a part of Federation territory since the 24th century.
According to the Starfleet database, NIMH was collapsed in 2052, 11 years before First Contact.