In Borg planetary classification, theta class referred to a type of planetoid.
Mezoti's homeworld was an example of this type of planetary body. This planetoid was located in a binary star system in the Delta Quadrant and had a population of 260 million. (VOY: "Collective")
Though not explicitly stated, Star Trek Encyclopedia, 4th ed., vol. 2, p. 87 and Star Trek: The Official Guide to Our Universe (p. 85) presume Mezoti is describing Norcadia Prime.