From the Greek, meaning 'Great Horn' -- these giant deer lived throughout Europe and even down into North Africa with related forms found in parts of China. They were pretty well on their way to extinction by the end of the Pleistocene Era, 11,700 years ago. Humans hunted them in the Paleolithic era and some posit this as a major factor in their dying out, of course some also think they died out because their antlers were just so freaking big.
Early paleontologists thought they were related to moose, on the strength of their size and the shape of those antlers. Megaloceros giganteus was found in large quantities in bogs around Dublin, and they have been called 'Irish Elk' despite their much wider geographical range and the fact that they are really not related too closely to either European elk or the North American elk. Their closest remaining relatives are actually much smaller deer, smaller, with daintier antlers by far.
I love that Megaloceros, the Irish Elk, features in current heraldry -- I think it's awesome that amid the range of everyday animals: roosters and salmon and bears, as well as mythological ones: dragons and hippogriffs and mermaids, there's room as well for real animals that havent been seen since the Ice Age.
Northern Ireland uses the Irish Elk in a configuration that makes a somewhat contested political statement, announcing its allegiance to England. The classic heraldic formula puts the royal lion of England across from its symbolic colonial spouse, a unicorn for Scotland, a stag once represented all of Ireland, the classic red dragon of Wales, and even, wonderfully, a penguin in the case of the British Antarctic Territory. (other notable pairings with the English lion: a Chinese dragon in Hong Kong, a tiger for one of the princely states that now comprise Gujarat in India, a gnu in one of the South African provinces, a Kangaroo in New South Wales, Australia.)