The couple that come to mind:
Remoraid (remora) - Octillery (Octopus)
Carvanha (piranha) - Sharpedo (Shark)
I guess this really only involves animal-based Pokemon.
One that is kind of pushing it is Rookidee (Chickadee) - Corviknight (Raven) I can kind of see how they would evolve though.
Pokemon like the Charmander line, (Lizard to Dragon,) don't count because they evolved to have wings and be more powerful.
Leave any questions in the comments