People who are used, either consciously or unconsciously, to enable the EIP's behavior or to punish the target.
Flying monkeys are the pawns in the EIP's game. They are the ones who do the EIP's dirty work, whether they realize it or not. The term comes from the Wizard of Oz, where the Wicked Witch of the West sends her flying monkeys to do her bidding. In the context of emotionally immature parents, flying monkeys can be anyone who enables the EIP's behavior or punishes the target, such as siblings, other family members, or even friends. They usually get involved because they care, but they care about their own peace and quiet (and will get punished if they don't), not about your wellbeing.
What's so frustrating is that the person who is doing the bidding of the EIP often genuinely believes that they are helping the situation. In reality they're stealing your safety by clearly choosing a side. Their side.