Getting the sheep in the pen
Go to the top of the field where the mountains are. Herd those two sheep in. The easiest ones will now be at the top of the field.
Quad biker
To stop Davey, the person on the quad bike, when you start chasing him, follow him. If you know any shortcuts to go towards Blacklock's house, use them. When you get to the end of each shortcut, stand sideways in the middle of the path. This might not stop him the first time, but keep trying and it will eventually work. You can also use the wide bits of grass to cut corners by galloping across them until you reach the path at the other side then stop sideways across the path. After three or four times you will stop him. To find the horses, go towards Blacklock's house but do not turn onto the lane that leads to his gate. Instead, continue straight until you get to the river otter and go through the chain. The first horse is hidden in the trees directly to the right of the entrance where you come in. After that, run round all the paths until you find them all.
To stop Davey, go over the first grassy part on medium gallop. Slow down near the path and stop in the middle of it. Estelle will say something. Use the nearest shortcut. When you are past the second part of path near there, go to medium gallop and do it again. If you need some endurance, make sure you have some. Trot while going near the second small path. You should have stopped him. Keep going straight when you have done this, then go to the river otter. Keep searching and you will find the horses.
Stop him once by going across the grass and stopping in the middle of the road. Estelle will say something. Continue until you can cut across him again. When you are in jumping distance of the road's center, press [Space] immediately followed by [Esc]. If done correctly, the game will game but the horse will keep jumping. Click "Continue" and you will have stopped him. A few attempts may be required.