Short answers, pulled from the story.
Gary Gygax assembled the catalog of creatures that became the backbone of Dungeons & Dragons while sitting at a desk in 1974. He drew from medieval bestiaries, Greek mythology, and science fiction films to populate his game world.
The Monster Manual arrived in 1977 with stat lines printed directly alongside monster descriptions for the first time. Gary Gygax wrote much of the initial content himself while expanding upon monsters found in earlier D&D supplements.
Tanar'ri demons roam the Abyss where the population is theoretically infinite in size and home to countless layers ruled over by demon princes. Balors originally called Balrogs push weaker tanar'ri around into makeshift armies for battle.
The fourth edition listed the tarrasque as an abomination classed as a gargantuan elemental magical beast. A living engine of death created by a primordial race served as a weapon against gods in that version.
Two horns projected forward from the top of its head making this creature look more broad and muscular than a Tyrannosaurus rex. The skin provided excellent armor leaving it immune or resistant to most offensive magic while regenerating damage quickly.