This usage isn't very new - in fact I've been being annoyed by it since the 1980s - but I don't think it's that old either. The OED, at any rate, doesn't have the phrase in this sense, but only to describe cattle that are sold while alive - which is what I'd always understood it to mean. My long-held theory was that "on the hoof" had been coined by analogy with "on the fly", which I certainly heard used quite a bit when I hung out with computer programmers in the mid-80s, to describe programmes that worked stuff out in real time. "On the fly" itself is a piece of thieves' cant, apparently, describing a way of begging; but I imagine this particular sense derives indirectly from baseball.
But why did anyone think that "hoof" was a suitable word to imply being light on one's feet? Something to do with hoofers, perhaps?