A scraprat is a "single" person of any gender who consistently sniffs around after other people's partners looking for sex/attention.
They come in two distinct versions; pet rats and plague rats.
Pet rats end up attracted to people in negotiated poly and/or open relationships. Sometimes they don't actually want to be someone's primary partner, and they enjoy having an erotic relationship without the burden of partnership. However often they are making do with being person <1 when really they wish they could be someone's special person. Trying to feed themselves on whatever scraps are left over from the primary relationship.
Plague rats on the other hand have no integrity... they often form friendships with someone they're attracted to then push the boundaries of flirtation to see what might happen. This of course does not excuse un-ethical behaviour on the object of their ratty affections! Plauge rats love drama and conflict, they're looking for an excuse to dash in and snatch whatever they can.
Another character common in the poly and BDSM communities is the "rat catcher" - a person who has a keen eye for scrap rats and likes to accumulate them. Feed them the occasional scrap of attention but it is to the Rat Catcher's benefit to keep them hungry and unsure... that way they'll never be confident enough to realise they can have more.