The Grizzelas were a species known to undergo hibernation cycles lasting at least six months.
In 2366, the Sheliak refused to allow the United Federation of Planets three weeks to remove 15,000 Human colonists from Tau Cygna V, which fell under Sheliak ownership as per the Treaty of Armens. Citing terms from the treaty, Captain Jean-Luc Picard named the Grizzelas as third party arbitrators to help settle the disputes. When Picard mentioned that the Grizzelas would not awaken for another six months, the Sheliak agreed to the original request of three weeks. (TNG: "The Ensigns of Command")
This species was only mentioned in dialogue.
Their name is possibly a reference to the grizzly bear, which are also known to enter a state of hibernation. Their homeworld was located in the vicinity of Tau Cygna V.
The Star Trek Encyclopedia, 4th ed., vol. 1, p. 319 incorrectly spelled their name as Grisella.