Octillery looks unrelated to Remoraid.
I think this argument has been played too much. I mean yes it's true that
some Pokemon and their evolutions aren't the most similiar looking of things but ALOT are. I mean Remoraid->Octillery, Magikarp->Gyarados, Pineco->Forretress and Feebas->Milotic. But most of those do have some basis. Machinery, Ugly Duckling, Weak grows strong etc. Mamepato and Swana have an abundance of differences between them. Besides the fact that Mamepato has generic bird written all over it, Swana has webbed feet while Mamepato has normal Pigeon ones.
Most the weird evolutions come from the
Water typing or more specifically, they're often fish. This could have something to do with evolution (not Pokemon evolution, could get put in jail for teaching it in the 50's evolution.) But to be sure I'll make sense of the odd evolutions.
1) Remoraid and Octillery are tough ones but they do have clues in their name that differ from almost every other Pokemon.
2) Magikarp and Gyarados. This one's really not that hard. I mean they share the exact same crown-shaped dorsal fins and whiskers. Not to mention the supposed origin:
3) Feebas and Milotic. This one isn't obvious physically but it's not to hard to figure out. Feebas is designed to be ugly and crappy so it can evolve into Milotic, the beautiful strong Pokemon that it is. Not only does it epitamize a very well known children's story, it copies the evolutionary chain of example number 2). It also follows the fish theme.
4) Pineco and Forretress. This is a harder one. (For me anyways) But based on appearance it's not inconceivable that they'd be of the same evolutionary chain. Plus they both could represent weapons (Hand grenade, turret.) Also, they both resemble tree nuts, and as they are based on bagworms, that makes sense.
In summarizing, Mamepato and Swana
could be of the same evolutionary family but at this point it's redundant to think so; besides their ability-which means little-there is nothing to suggest that Mamepato will evolve into Swana.