The Caatati were reptilian lifeforms who, until the early 24th century, maintained a proud civilization. At that time they were assimilated by the Borg, except for 27 ships. These escaped carrying a small number of survivors but were forced to wander throughout space, begging where they could, but more often than not being driven off.
In 2374 a Caatati ship encountered the USS Voyager and were given stocks of food. A little later, they found Voyager's warp core after it had been dumped and claimed it. They then tried to use it as a bargaining chip for more supplies.
Instead, Seven of Nine offered them the lost thorium replication technology, which she recalled from her time with the Borg. The Caatati gladly agreed.