Do you feel like creating an army of Silverfish with around 250 hearts each or a Wither with only one heart? If so, you're in the right place! I have found a way to create mobs with the amount of health you want! Now, if you already knew how to do this, then I suggest reading something else. ON WITH THE TUTORIAL.

First, you will need a command block. You can get one by typing either this:
/give @p 137 1
or this:
/give @p minecraft:command_block 1

Next, place the command block anywhere you want then right-click on it. In the prompt, type this:
/summon mobname x y z {Attributes:[{Name:generic.maxHealth,Base:#}]}
You can replace mobname, x, y, z, and # to whatever you want.

An example of usage can be this:
/summon Pig ~ ~1 ~ {Attributes:[{Name:generic.maxHealth,Base:1000}]}
This would create a pig with 500 hearts.

Hope you enjoyed this little tutorial. :I
~Maedmagor

NOTE: You probably knew this already, but 1 health point is equal to a half of one heart.