Behemoth.
The word had been a part of his vocabulary for as long as he could remember (which was pretty long, considering his first flashes of memory turned out to be from a trip his parents had taken him on when he was two-and-a-half). He liked to roll it around on the tongue of his imagination as he waded bare-legged through the murky waters of the creek behind the house. There would never be room for any creature worthy of the name in such a winding ribbon of water, but if he imagined himself very small then the scale made it feel possible.