What monsters were cooler in the older editions?

[Metastachydium]

It's kind of a cheaty answer, but eladrin. Eladrin were way cooler as the CG exemplar race in 2e than they are now as elves-with-extra-feelings in 5E.

…and even though barring two types, all of them were BoED material in 3.5 (which, needless to say, automatically erodes some coolness factor off of stuff), in 3.x too. I thnk this one really was 4e's fault.