What is the chance that a Pokemon from a horde battle will have its hidden ability?

Question

Please answer with an exact chance.

All answers that do not answer with an exact chance and a link that can back it up (or some other evidence) will be hidden.

Comments

That's my point, and we don't know the chance of that either I believe, that's why i'm saying that if you're gonna wait for it to be released, well that day probably won't come :P

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Yes it will. Hackers are like arsonists, they spray foundation (rumors) and then spread the fire (truth).

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Ok, in that case let's wait for these hackers that have already been taking 3+ months for such a thing, i think they'd much rather hack for better stuff but alright I suppose if you wish to wait and see if it happens.

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I can't find it. I've looked in this game's code for a total 6 hours or so over time.

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You have the code? o3o

Answer 1

Now, I know you probably want an official source answer for this question, so feel free to hide this if you don’t think this is an acceptable answer.
However, I really doubt we will officially be told the exact chance, since we haven't already, and so I have decided to devise and experiment to answer this question.

The Experiment:

I looked around for Pokémon that appear in hordes that I could easily determine whether or not they had their hidden ability. In the end I settled on the Whismur hordes in Connecting Cave for this experiment.

I chose Whismur for several reasons. Firstly, they are common, meaning I won’t be spending half an hour to get just one horde. Secondly, they have Soundproof as their only regular ability, and Rattled as their hidden ability. If you are thinking “how the heck do you easily test for Rattled?”, then my response is I didn’t need to. Since Soundproof is their only regular ability, those without Soundproof must therefore have Rattled. While Rattled may take some time to test for, Soundproof is very easy to test for. Simply have your Pokémon use Growl, and a message will flash up telling you how many have Soundproof. Since Growl affects every opponent, it means I only have to use it once to tell how many have Soundproof, and how many have Rattled.

My Growl user was my trusty Kricketune. Why? Well, that is mainly because Kricketune was the highest levelled Pokémon I had in my box that still knew Growl, which is generally considered rather useless.

I ended up testing 200 Whismur hordes, which gives a total of 1,000 individual Whismur.

The Results:

So now I have established my methods, what were my results? Well, 952 Whismur had the ability Soundproof and 48 had Rattled. Since I am only interested in the Hidden Ability for this, then 48/1000 = 0.048 or 4.8%

The Final Answer:

So from this experiment I got the chance of a Pokémon in a Horde with its Hidden Ability is 4.8%. Since GameFreak seems to like round numbers, I suggest the actual answer is probably 5% then.
If anyone else wants to try this experiment (either with Whismur, or another horde of your choice), then compare your results to mine and see what you get. Since this is statistics, the more people who actually do an experiment like this, then the more accurate our answer will actually be.

Comments

And Kiraria also said 5%
And I found 10 Axew Hordes and 4 of them was HA then I took 5 Whismur hordes and 3 of them was HA

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What number is your Trainer ID, and how many shinies did you encounter, if any? im doing another experiment about the correlation between the given Trainer ID number and the chances of getting s shiny pokemon, so this information would be very helpful.

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Wow, this was a while ago. It was on my Pokemon Y game. My trainer ID was: 35734. I got no shinies :(

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thank you so much!

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How many shiny whismur did you encounter

Answer 2

According to this statement:

It took me two hordes to find a Plusle with Lightning Rod, seven hordes to find a Zubat with Infiltrator, only two to get a Tailow with Scrappy, 5 to get a Whismur with Rattled (I corrected it from Scrappy) etc.

The chance to found a Pokemon with Hidden Abilities is 2+7+2+5/4 = 16/4 = 4, a HA Pokemon in 4 Hordes. Since 1 Horde consist of 5 Pokemons. It means 1 in 20 Pokemons. If this converted to a percentage, it's mean 100/20 = 5%. Just like MeloettaMelody said.

There is a way to check if a Pokemon had HA or not by this statement:

I usually lead with my Synchronizer and I switch into a Pokemon that is tough and tanky that has either Entrainment or Worry Seed as a move. That way I can check abilities one by one, and if none are the one I am looking for I run.

You should not catch them blindly, you really need ways to check abilities in battle with moves in order to increase your chances at catching the right one in the least amount of time.

Thanks for the advice, I went with Blastoise's suggestion and trained up an abra I caught in the friend safari to a level 42 kadabra with role play, so that I could see what ability each one in the horde had. Took me only 3 horde encounters until I found one which had hustle :D Gonna start breeding it today to try and get some more with HA, hopefully the fact that its male won't make it too hard.

Source

P.S: Sorry I can't confirm it since I still haven't own either Pokemon X/Y also I didn't owned 3DS.