Video originally uploaded onto Youtube in 2012.
Zelda OOT
Meme went viral in 2015
https://www.youtube.com/watch?v=0FLQ_h_LaqA&index=2&list=RDfWb2TwRDHSY

It also spawned the upbeat strange funkish forest feel meme.
Archives show that it was shared in cringe threads on 4chan's /v/ board as early as April 2015 and kept being shared. Due to the archive going down information on this meme during the June-Oct 2015 gap is missing but after that gap, the video had already reached viral status and had threads dedicated solely on that video.
In 2016 the creator, MattMVS took down the videos due to the community laughing at the content but it was quickly reuploaded onto youtube.

During that june-oct gap the meme(s) quickly became a widespread joke on youtube (such as videos relating to the Forest Temple theme), funnyjunk etc.

sources
top comments relating to Eh Reh on various videos on youtube date back to two years ago in 2015

https://boards.fireden.net/v/search/text/matt%20mvs/

EH REH

First appearance? https://foolz.fireden.net/_/search/text/o6nJysVtCrM/page/2/

https://boards.fireden.net/v/search/text/upbeat%20strange%20funkish%20forest%20feel/

2016 he took the videos down, emptyhero commented https://boards.fireden.net/v/thread/348990848/
MattMVS the creator threatened suicide but subsequently returned in another video saying that life was worth living.

https://boards.fireden.net/v/thread/314900717

https://boards.fireden.net/v/search/image/3EGzjI0TuqXX7qoINAVBZg%3D%3D/page/4/

Explanation thread, perhaps viral 2015 july but video existed since 2012 https://boards.fireden.net/v/thread/348999284

The first appearance on /v/ of what I've found so far was 2015, but the meme was stuck in cringe threads. Then foolz died after june and when it came back, the meme became an instant classic. Vid was originally posted on 2012 but I don't remember seeing it back in 2013 or 2014. So did EH REH? just go viral around July 2015 then?

It seems /mu/ might have been somewhat related to the cause of the boom of Eh Reh. The archive was still up and active at the time and threads dedicated to it occured as early as July 2015
https://archive.rebeccablacktech.com/mu/search/text/eh%20reh/page/2/
https://archive.rebeccablacktech.com/mu/search/image/Cb%2FAB0cnzjd55972IATbnw%3D%3D/page/2/