Yellow bill oxpecker
The Oxpeckers are found in sub-Saharan, savanna and open grassland habitats. It use to ride on top of Mammals and clean them from ticks, flies and other parasite in exchange for the ride they get.
Some researches are going on these days, trying to determine if the Oxpeckers themselves are parasites (it was observed how they open old wounds in order to suck blood) and not really helpful for the cleanliness of the mammals.
There are two species of Oxpeckers, here you can see the Yellow bill (Buphagus africanus), riding on Zebra.