I am quite sure that a lion is a symbol of strength, and if I am not mistaken (which I may well be), the gryphon too is a symbol of strength, but with intelligence added on (symbolised by the eagle head, I think). I have never seen a chimera in any heraldry (though that means nothing; I haven't really paid much attention to that sort of thing), but in Greek mythology it is simply a monster, an amalgam of a lion, snake, and goat. The last one leads me to wonder if the symbol is even a true chimera; perhaps it is an altered version. In any sense I think the reason for this is a symbol of fear and monstrous power in the Empire.