If you try to spawn a pig,for example,with the command
"/summon Pig

{NoAI:1}

" he won't move,he's not affected by gravity,doesn't eat etc...
But if you try with Squids they are affected by gravity and swim if you spawn them in water... Command:"/summon Squid

{NoAI:1}

"
And if you do the same with endermites (/summon Endermite

{NoAI:1}

) he won't move,but after a short period of time he'll disappear
for the ones good with commands: I tried to put the tag

{PersistanceRequired:1b}

,but still doesn't work
it can't be explained with an image,sorry