As referee I would allow a mobile target like a wagon or saddle provided 1) the caster was very familiar with the target in question and 2) the caster knew generally where it was (i.e., within a few miles).
If the caster had no idea at all where the wagon or saddle was, the spell would fail.
"The fear of death, its risk each time, is one of the most stimulating parts of the game. It therefore behooves the referee to include as many mystifying and dangerous areas as is consistent with a reasonable chance for survival." - J. Eric Holmes