Ah, well, if you consult the actual official
wiki, you'll find Chimera, Shard of Focus, Fate Egg, Devonian Dragon, Scarab, and Immortal are exempt from the list of possible mutants. The first two make balance and flavor sense. The rest seem to be arbitrary and could either be a fluke in the code or a choice by zanz long, long ago. Perhaps the latter four were deemed unchangeable by entropic forces (Phase Dragon was originally not immaterial like Immortal).