DamageIndicators

DamageIndicators is a mod by rich1051414 that adds a "Mob Portrait" to the game. A Mob Portrait is a small box that displays creature info, such as a close-up real-time animation of it, health stats, status effects, and creature type (hostile, neutral, arthropod, etc.), whenever you mouse over a creature. DamageIndicators also adds a neat effect to creatures whenever they take damage, or are healed. When a creature's health is altered, a small number displaying how much health was lost/gain will bounce off of them. The color of the number determines whether health was lost or gained.

Config
Pressing period ('.') will open a simple config menu where you can change the GUI scale (default 75%) and toggle the portrait, skin (i.e. any texture pack GUI for the window), the showing of potion effects, and pop-off damage indicators.

By pressing 'advanced' you can adjust the position of the window on the screen and change its size for every mob individually.