I volunteer at a cat refuge. I recently brought home two kittens who seemed at risk of dying from failure to thrive—one of them had stopped eating and expended any energy he had climbing the walls trying to get attention from the volunteers. His brother was also small and weak, probably because the other kittens were big enough and strong enough to keep him away from the food. After almost a week, the little guy is finally eating and seems a little bit more energetic. So I bought one of those feathery things on a stick and dangled it in front of this tiny, weak kitten. HE WENT NUTS! He grabbed it, growled over and over again and would NOT let go. I’ve never seen anything like it (although, to be frank, I’ve never raised kittens before.) This kitten is just as loving as you could possibly want, let’s his brother knock his socks off, has never scratched or bitten anything and suddenly he goes from Dr. Jekyll to Mr. Hyde. I finally pried the thing out of his jaws and hid it. He spent the next 10 minutes looking everywhere for it. I’m afraid to bring it out again. Does anyone know why this happened, if it’s normal, if I should try it again, if I should worry…
BTW, my little foster guys are now my newly adopted kidlets.
Yeah, he’s just being a kitty. Cat play mimics hunting and fighting behaviors, so it tends to seem…aggressive to the uninitiated. That’s why cat toys are so often small things that make skittery sounds and movements, to mimic prey.
Wait till he feels good enough to start rolling around and kicking to disembowel the feather.
Yes that is totally normal.
Some cats have fetishes for certain items. When one of mine was young she liked socks. She would drag them around the house and growl at the other cats if they came near it.
If you don’t mind the kitten being overly protective of the toy then by all means let him have it. If he comes too protective then that could become an issue. Some get so fixated that they won’t do anything else but protect and play with their “kill”.
You may just need to find another toy that he likes to play with but does not go ape over.
One piece of advice–I’ve seen kittens play so hard with toys that they panted and collapsed. Monitor their play and put the toy away after a few minutes (this not only keeps the cat from over-exerting, but keeps the toy “fresh” and intriguing). Make sure there’s easily accessible drinking water as well.
Kittens at 12 weeks are little balls of energy. If you could somehow convert 10% of the energy produced by all the puppies and kittens of the world, you could power … I dunno, but I bet it would be big and bright.
So you’ve got this little kitten who’s been somewhat deprived of the usual features of a happy kittenhood. Now he’s feeling just a little bit better, just enough to start feeling a tiny spark of that insane energy … and suddenly, he sees … a toy! OMG! The joy! The feeling of *playing *for once!
Of course he’s going a little nuts. It’s what happy kittens do! 
I think the only advice I have is to offer another toy. And another! Try a big, strong fabric ribbon that wiggles well. Crumple up some foil or paper into a lighweight little ball and the roll it across the floor for him - lots of cats like that.
Enjoy the little fuzzbutts. And take lots of pictures! They grow up so fast. <sniff>
(Note to kitten newbies: catnip toys to a little kitten are just, well, toys. They don’t respond to catnip until sexual maturity, and some never respond much at all.)
Echoing what everyone else has said, it’s normal and adorable. Also, there is something major missing from your post. It’s a cat thread and we have rules.
My big lazy lump-o-love never has been and never will prefer playing to just hanging out and lovin’ on you. EXCEPT, when he was a tiny kitten, I brought out a fur-covered mouse toy and got the same result the OP did. He would growl (cutest thing) and not let anyone take it away.
I am a total cat/kitten virgin (except ones when I was a kid and since I didn’t take care of them, that doesn’t count) so I am at a loss for proper cat thread etiquette. Care to give a girl a clue?
Pics, please! 
When I was young I had a full grown cat and it went nuts over a feather duster.
She would grab it so hard it’d fall out of my hands, she’d wrap her entire body around it and rolling and wrestling with it. She loved to fight the feather duster
Of course. Dopers, meet Tigger (as in Crouching) and Dragon (as in Hidden). Tigger and Dragon, meet the dope. We’re hoping Dragon, the black one, grows into his name. Even after a week, you can still feel every bone in his spine, ribs and hips.
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I know this is a cat thread, but my 3-yr old Bichon doggie does this with certain treats or bones. He’ll take it and hide it in a corner and camp out there, and growl at the other dogs in the house if they come near it. I call it “hoarding.” I usually take it away from him, or break it and then he eats it. He usually does it when it’s too big for him to eat in one bite.
Have y’all seen this behavior before?
Ta Da indeed!
Awww … great pics. Cute lil’ babies.
I forgot to mention a really great cat toy that Mr. Horseshoe came up with - he tied one end of a sturdy fabric ribbon very tightly around a champagne cork. The end of the “stem” of the mushroom-shaped cork is fatter than the middle, so it stays put. He did that months ago and it’s still holding tight despite being one of our cats’ favorite toy. (All the fun of a wiggling string, plus a little weight to make it swing around better, plus I think the spongy texture of the cork is fun to dig claws’n’teeth into.)
Pop open a bottle of cheap bubbly to celebrate your kitties’ victory over death-by-failure-to-thrive and make 'em a toy!