You want to know what a grick is, boy? It's an abomination from another world. A planar maggot!
Just as they've been called by conjurors, a grick is considered a "planar maggot" which slips between worlds by "hitch-hiking" on summoned outsiders. They typically come into a realm by living on an elemental. Elementals lack the ability to become aware of a grick's presence, though others are swift enough to regret such incapabilities.
With the visage of a slug and octopus hybrid, a grick can spit acid up to thirty feet away and dissolve most organic things. They then use their tendril-like tongues to soak up the dissolved flesh and sustain themselves on it. Gricks chase anything with a beating heart, but otherwise ignore the unliving.