Hmm--I've not heard this one before, but 'by the fetlock' makes me think of 'by the short hairs'; in other words, the grabbee cannot move.
Hmm again--that really doesn't make sense, unless you're talking about making time stand still, which of course can't be done.

The only other interpretation I can think of is along the lines of having a lot of something, as in, "Oh, I have books by the truckload". But 'by the fetlock' wouldn't make sense in that way either: even just one horse only has four fetlocks. [shrug]