Does a Legendary Card obtained as an arena reward resets the 'Pity Timer'?
It's a widely accepted idea that there is a 'pity timer' when opening card packs in Hearthstone, meaning that after X number of packs opened without a Legendary card, your next pack will always contain (at least) 1.
After a recent 12 wins run (Yay!) My reward contained 1 pack, a bunch of gold and a Legendary card. At that point I thought maybe I hit my pity timer, as it had been a lot of opened packs since my last Legendary.
Does a Legendary Card obtained as an arena reward resets the 'Pity Timer'?
Solution 1:
Legendary Cards acquired from Arena rewards do not reset the "pity timer". This is because the pity timer is only a concept that applies to Legendary Cards acquired through pack openings. As Arena rewards do not fall within that category, no pity timer is effected.
Similarly, the free Legendary Card that you get from the Welcome Bundle does not reset the pity timer.
It is also interesting to note that the pity timer counts independently for each type of card pack. Thus getting a Legendary Card in the newest "Mean Streets of Gadgetzan" card pack will not reset the pity timer for getting a Legendary Card in a Classic pack. Source
Hope that helps.
Solution 2:
I have never heard of Arena rewards being on a pity timer - I believe they are completely random.
What I had read about the Welcome bundle was that the bundle Lengendary had no effect on chance to get a Lengendary from packs. It's a safe assumption that the same rule holds for Arena Legendaries.