There are many similarities:
Pokabu does not look so radical from Stage 2 if it were bipedal;
note the transformation of its' tail & ears throughout the three stages;
colour scheme remains throughout all three evolutions with minor differences.
The tail goes from a red orb to a fluffy black scruffy something.. Then in the final evolution it goes to really scruffy and spikey. Where did the red thing go to? Makes no sense.
The ears, yeah, that's something all right.. But what about that yellow streak it has going up from it's nose? Totally gone, and nothing similar noticable in the evolutionary line.
Mijumaru: Sea shell on chest > Stage Two's leg plates > Stage Three armor;
note the tail fin transformations from Stage 1 > 3;
note the whiskers from Stage 2 > 3
Those two shells are on its belly, not its legs.. Yeah, and about that armor, Mijumaru uses the shell as a blade, now how can that third thing use blades at all? That doesn't make much sense doesn't it?
The whiskers is probably the only thing that stayed relatively the same and regular growth throughout the stages. I can see the REAL final evolution being a bipedal old ninja with those kinds of mustaches, but this thing is not the real thing.
The grass snake's similarities are obvious enough.
True..
As you said, compare Torchic and Blaziken. Nobody could have expected that. Just enough reason to support Mijumaru and Stage 3.
Uhhh.. Blaziken is fairly logical if you don't mind me saying: Chick, Chicken, Coqfighter. Combusken held most of the things Torchic had intact, as in the the head plumage, the beak, the colors, and when Torchic evolved they were just inversed and it just made for a larger, more grown out of its chickphase chicken. It also already gained that Fighting type at that time, so transitioning into a coqfighter made just a lot of sense. It never changed body form or anything. No starter ever did.
Torchic: Chicken, Chicken, Chicken
Mijumaru: Otter, Otter, Narwhale/Walrus/Otterish
The change from Torchic to Combusken is attributed to natural growth from a chick to chicken, and since there was already a set up for the third stage with the fighting type addition, it wasn't as far a stretch, au contraire regarding Mijumaru's full stage, since it just seems like a complete revision of the thing. With nothing bar, some shells (barely the same even) and whiskers...