Easy to find if you know where to look.
There is an easy Shiny Mareep up for grabs in Pokemon Legends: Z-A, but you need to have hit a certain point in the campaign to get it, and finish a side mission. In this guide, we will tell you everything you need to know to add this Shiny Mareep to your Pokedex.
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Where to find Shiny Mareep in Pokemon Legends: Z-A

To get this Shiny Mareep, which appears in Wild Zone 1, you will first need to play up to Mission 8 of the main campaign. Once you finish that mission and hit Rank V, you will then need to chat to Lida, beat her in a battle, and then more side missions will populate.
At the northeast entrance to Wild Zone 1, you will find an NPC standing there with two Mareeps. She will have heard a rumor of a Shiny Mareep in Wild Zone 1, and will ask you to find it for her. Go into the Wild Zone and make your way to the section of fence blocked by two rocks, with the small pool of water on the other side.
Break the rocks if you need to, then head in and climb the ladder to the roof. From there, turn right and climb to the next roof, and this is where you will find the Shiny Mareep. A cutscene will start, and the NPC will appear. She will just be happy to have seen the Shiny Mareep, but you can catch it. I just used a Great Ball and waited until it was looking away to throw it, and that did the trick.
For more guides and news, be sure to check out our Pokemon Legends: Z-A page.