Poliwhirl = Tadpole.
Politoed = Frog.
Tadpole -> Frog.
Does that explain it? :P
Phanpy = Small Elephant.
Donphan = Larger Elephant.
Simple upgrade evolution, stick a cut car tire on its back and you get Donphan. The end.
Anyway, here's a few:
Grovyle into Sceptile. Whilst Sceptile would make sense as an evolution of Treecko (It is after all really just a Treeckosaurus Rex), it makes utterly no sense when put immediately after the sleek, somewhat-dragonish Grovyle.
Dragonair into Dragonite. AGH. How can something as graceful as Dragonair can evolve into that winged Barney recolor?
Octillery and Remoraid don't make sense either. I don't think that Octillery is a 'true' octopus, but rather a fish that has taken the form of an Octopod. A fish simply cannot turn into a Mollusk.
And as Ivee mentioned, The cuteish Tyrogue into the mutant Kicking Spud of a Hitmonlee or the Beyblade Wannabe Hitmontop.
But the craziest thing ever to grace Pokemon is Clamperl. Uh, explain to me how can a Pearl (the real Pokemon is the Pearl, NOT the clam) can evolve into an Eel? or a Lamprey/Hagfish/Just-Whatever-The-Hades-Gorebyss-Is-Supposed-To-Be?