The Torah says... we may not harness an ox and a donkey together to pull a plow. Our Sages teach us that it is forbidden to pull a circus wagon with an elephant and two giraffes harnessed together (as described in Dr. Seuss' "And To Think That I Saw It On Mulberry Street" - his first children's book, by the way). Is this Rabbinic extension of Torah Law? No, it is not. It is the transmission of the Oral Law that teaches us that ox, donkey, and plowing are all examples, and that the tying together of any two animals for any function is a Torah violation.
For more on Parshat Ki Teitzei and its Mitzvot galore download the podcast.