'theprey' is a compound word denoting the conjunction of the and prey.
The is taken in this context to mean a person.
Prey refers to the fact that this is a person who conceals their true identity through a series of false identities presented online, aliases and assorted other smoke and mirrors designed to obfuscate anyone who tries to find them. They fully understand their role in the hunt - that is, to be the hunted. In their preremptory fabrication of multiple aliases, they have premeditated that this is the position they would prefer to be in in a relationship. Compare to the submissive partner in a DS relationship.
Notes about theprey variants are:
Some enjoy the process of being hunted so much that they will run too far: the predator will lose interest or become so tired by the chase they are forced to give up. These are called thepreytoofast.
Others will begin to use the chase to gather multiple predators, in the knowledge that each in the course of the hunt will be too distracted by the quarry to notice each other. They often end up hurt in the fight at the end, because when a predator finds another who has already cornered his quarry, he will fight for it. These are called thepreyinpain.
I met a theprey last week. He's got four girls following him already, but plays innocent all the time.
She's such a theprey! I've fought off six guys so far and there's still more before I can even approach her!