I mentioned that Lambykins was giving Terzo the business while he was trying to show her at fair... his arm bears the proof.
See the little points on her hooves in that bruise? She knocked him down and stepped on his arm while he was trying to position her with her legs lined up in a square, known as "setting up" the sheep. She took advantage of his crouched down position and went for the escape. He didn't choose her "on the lam" convict costume for nothing!
Being the well-trained farm kid that he is, he never let go of the halter, though he did have a hard time keeping her under control once she sensed freedom was at hand.
Primo eventually stepped in and held her head (which goes a long way toward controlling a sheep) so Terzo could concentrate on the proper placement of her legs. At his level, it didn't matter that he had help.
It takes a lot of practice to set up a sheep properly! The advantage of having a cooperative sheep can't be overstated, either.