Generally speaking, this is a term of endearment for a male, or female, who is the submissive in a dominant/submissive (hereby referred to as D/s) relationship. D/s relationships come in many different flavors, to include Master/Mistress and slave, though more common is the Owner/pet variant.

Specifically speaking, a Sexpet is usually referencing the pet, who's primary function is relief and enjoyment for the Owner/Mistress/Master through sex, foreplay, and other intimate interactions.

The pet usually sees his/herself as an animal, usually a domesticated housepet, but some see themselves as more varied creatures like a fox or even a leopard.
In addition, usually the pet is nearly dependent on the Dominant's initiative, as the sub tends to have no drive of his/her own.

(noun) A successful type of theft which results in an acceptable, impressive and rewarding payday for the protagonist.

Term of endearment referring to casual, but reliable sexual partner. Complimentary, and intended to be sweet, but also slightly demeaning.

See also: booty call, friend with benefits, fuck buddy