Hmm. Well, nothing really jumps to the forefront of my mind… I’m scrolling down Smogon’s Little Cup tier list (that being the only place I could think of that just lists all the unevolved Pokémon) looking for possibilities but I’m having difficulty. I mean, I like the cute ones, don’t get me wrong, but in most cases I find that that’s outweighed by the sense of realisation of potential that comes with evolution. I’m half with you on Magby, I suppose, in the sense that I do like Magmar but Magmortar looks pretty stupid. Perhaps Glameow a little, because Purugly is just so grumpy and surly-looking. Koffing, definitely, because it’s a shame that Koffing is so happy and Weezing so… lung-cancer-y. Patrat, I guess, insofar as Watchog is that much more unspeakably nightmarish. And… Ralts, but only to spare her from the ravages of Rule 34. The internet is a dark and terrible place.
Regarding the tiyanak… hmm. Well, the biggest objection I can see is that tiyanak normally do look like children. In the absence of any strong, specific tip-off that this is what they mean (e.g. in the name), it seems plausible that there could be some other similar myth, closer to Japan, that I just plain don’t know about. But it makes sense; I don’t think it can be ruled out.