Let’s adopt the “rightward is positive” convention for the motions below

Since the sheep is moving rightward and speeding up, its velocity is represented by more and more positive numbers.

Since the velocity is becoming more positive, the acceleration is positive.

–

Since the sheep is moving leftward and speeding up, its velocity is represented by more and more negative numbers.

Since the velocity is becoming more negative, the acceleration is negative.

–

Since the sheep is moving rightward but slowing down, its velocity is represented by less and less positive numbers.

Since the velocity is becoming less positive, the acceleration is negative.

–

Since the sheep is moving leftward but slowing down, its velocity is represented by less and less negative numbers.

Since the velocity is becoming less negative, the acceleration is positive.

In a nutshell, the speed increases if the velocity and acceleration have the same sign, and decreases if the velocity and acceleration have opposite signs.