I'm interested in compiling some lore on the gray render, but the Wikipedia entry dates them no further than D&D 3.0. Did they exist before this? Did they exist before D&D came along or were they a 3.0 invention?
No, they were invented by Wizards of the Coast. There are a number of creatures in 3.x that never existed before and have no lore in either real myth or in earlier editions of D&D.
After converting the classic AD&D monsters that they wanted to keep and figuring out their CR, they found that there were CR numbers that had too few creatures for variety. They filled these with new creatures. The Krenshar is one such creature they've acknowledged exists only to make the game's math work, but there are many and the Grey Render is likely another of them.