Hybrids are a catch-all term for the phenomena occurring when either a human or a creature is hybridized with another creature. They aren't understood very well, and the human ones aren't treated very well at all. The more human Hybrids have the option of using a Power Limiter to appear fully human and become Tamers, but the less human and Exiled Hybrids have to settle for being treated marginally better than a creature. For this reason, the Restricted Area is rumored to be a gathering place for these strange creatures, among other things. There are numerous warnings about the safety of the place for non-Hybrid Tamers, but a Hybrid Tamer should have no problems looking into the ruins to discover new things.
Human-based Hybrids have no rarity for species, although Sorila seems to be extremely rare regardless.
Curiously, there's a prominent Hybrid working in the Hybrid Resource Center, and a rare Sorila at that. The Hybrid Resource Center handles Hybrid registrations, as well as the distribution of Power Limiters.