here is a highly offensive one.
you take turns completely the following verse
Beastiality's in Boys, Beastiality's in!
I gave a good [term meaning sex] to a [animal name rhyming with term used], Boys!
I gave a good [repeat term] to a [repeat animal]!
OHhhhhh!!!!
Repeat indefinitely or until beer runs out.
example:
{everyone sings} Beastiality's in Boys, Beastiality's in!
{just one person sings} I gave a good ram to a lamb, boys,
i gave a good ram to a lamb!
{everyone sings}OHhhhhh!!!!
repeat, taking turns by going in a clockwise fashion around your table.
So, the object is to have fun, be creative, and if you screw up, you must drink! Oh and you can't repeat rhymes so it starts to get interesting after a while.