(Breaking this number down a little further: 70 creatures last appeared as D&D critters, 743 creatures as AD&D critters, 1377 as AD&D 2nd Edition critters, and a whopping 6480 as D&D 3.X monsters.)
Assuming no other new creatures are created between now and 4e, let's hope 4e introduces exactly 1330 new monsters to bring us to a nice, even 10,000 total.