If this was posted already my bad.. focus macros.
This is a 2 macro step.
/script SetRaidTarget("focus", 1)
/cast [target=focus] Polymorph
How it works.. target your sheep click macro 1, when you are ready to sheep click macro 2. Once you start DPS'ing your target you'll never have to click off your target to resheep, just click macro2.
This can be used on other classes as well such as banish and whatnot.. enjoy.