The colored line with the slider is the happiness bar. Happiness values reach from -100 to 100. Mousing over the happiness bar shows the exact happiness value. The face above the happiness bar will reflect the mood of the creature. Mousing over the face shows it's mood and it's likelihood of attacking you. Prehistoric creatures have five different moods based on what their happiness value is.
HAPPY: 100 Happiness Points Edit
The highest happiness value a creature can have is 100, making happy the best mood a creature could possibly have. When happy, a mob will not attack you unless provoked.
CONTENT: 50 to 99 Happiness Points Edit
There is no difference between behavior when content and when happy. When content, a mob will not attack you unless provoked.
CALM: -49 to 49 Happiness Points Edit
Calm is the default mood for all prehistoric creatures and also the largest happiness point bracket. Calm creatures will not attack the player or other mobs unless they are hungry or provoked.
SAD: -99 to -50 Happiness Points Edit
There is no difference between behavior when a mob is sad versus when it is angry. When a creature is sad, it will attack on sight.
ANGRY: -100 Happiness Points Edit
The lowest happiness value a creature can have is -100, making angry the worst mood a creature could possibly have. When a creature is angry, it will attack on sight.