Even 5,000mAh batteries in smartphones are not that uncommon these days, but they are fairly rare and mostly cater to the niche gaming community. However, I do not recall any smartphone available in the current market that has a 6,000mAh battery! That's a tablet-grade battery we are talking about here!
According to a leak, one or more models with a 6,000maH battery will be released soon and most reports are suggesting that it will be either the Galaxy M20s or M30s. There's even opinion that both of the upcoming phones will feature the 6,000mAh battery, but I have my doubts about that one though. Most likely, if it is a genuine leak, the M30s will sport the huge battery, while the M20s may not even exist.
The #GoMonster hashtag accompanying the image might suggest that Samsung is targeting this one towards the power users. The media buttons embedded in the image most likely indicates long video and music playback capabilities, which is very likely if the phone really does have a 6,000mAh battery.
Keep in mind that the photo is supposed to be a leaked teaser, rather than being an official one, so the Galaxy M with its mythical 6,000mAh battery may not ever come out. Then again, rumors and leaks are surprisingly accurate these days.