get WinMPQ, open war3x.mpq, extract the nightelfabilityfunc.txt file, find the starfall entry, change it to the sheepfall animation, import into map under "units/nightelfabilityfunc.txt".
uhhh ... i believe that there is a specific sheepfall art, where a sheep falls from the sky and lands on the target. Even if there isn't one in the usual MPQ, Fireaarro made a flaming sheepfall art thingy some time back, so that can be used.
There's a falling sheep that's related to Polymorph--it's the one used in the bonus credits in TFT. I can't remember the path right now, but I believe its something like Abilites\Spells\Human\Polymorph\PolymorphAirDone... look it up :D