Symbiotes

Symbiotes are life-forms from across the Veil that must attach themselves to sufficiently complex plants or animals in order to survive. In return for bonding with them, the Symbiote can grant its host increased strength, biological weapons, and heightened senses. 
The Symbiote possesses a mind of its own and hosts to Symbiotes must learn to live with two voices in their heads, their own and the Symbiote. Also, Symbiotes posses powerful, primal instincts, so those attached to them often end up acting far more aggressive and selfish. Kinda like a wild animal.