Most baddies have a rare spawn version. It's always fun when you see one. I just wish they dropped something better than a gold gralat (and it's not even a 100% drop rate..).
Anyway, there's also a rare spawn version of the bat in the railroad caves. It's kind of confusing though. For a while, there were two versions, the bats and the skull bats. Then they took out the skull bat and made it the rare spawn version (I can confirm this because I got a gold gralat from one a while back). But now the skull version spawns regularly in some spots, so there's no differentiation between the rare spawn and the regular skull bats.
And regarding the bombchu, I never knew they actually blew up sometimes until recently. I found one and upon attacking it, it exploded. That caught me off-guard. XD
Oh and golden blobs very much do exist (you never seem em because they spawn in such a populated area, so the odd time one does spawn, it's killed pretty fast). The giant golden blob, however, is a rumour. I've heard of it. I've heard stories. But I've never seen proof of its existence.