This seems very legit for some reason.. and this is coming from a usually very skeptical person. Here's some more info, apparently horses, among other things, are in. Might just be a well made early April prank though. I'll take down the link if the mods ask me to regardless:
EDIT: Actually, nevermind. "Playable demo on E3" got me thinking it's bullshit. Rockstar would never do that.