The Rhinoceros & The Oxpecker
As formidable as a rhinoceros may be, it can be quite sensitive to insects in the small fissures of its hide. Parasitic insects are often found burrowing into these cracks causing itching and irritation. Another potential weakness in the rhinoceros is its poor eyesight. The rhinoceros has found a friend in the oxpecker bird - the bird plucks out and eats the burrowing insects, and flaps its wings and squawks if it sees danger approaching.
This is an example of a mutualistic symbiotic relationship; the rhino stays free of pests and its sense of danger is enhanced by the bird's warning, and the oxpecker gets a free meal and protection for its vigilance - both species benefit from the relationship.