Mesprit is at Lake Verity but when you talk to it it runs away... You have to use your Marking Map Poketch thing, which is number 13. If you don't catch it straight away it runs and so I was advised to use a Pokemon that has a move that keeps it from running away. I'm still trying to catch it because I don't have a Pokemon with a move that stops it from running away -.- They said something like Golbat would work. To find Mesprit you just have to go to a town like Sandgem or something and keep going in and out until The mark on the map is in your route. Then just try to find the patch of grass that Mesprit is in... Hope I helped!
Mesprit is in the underground lake.
After you've defeated Dialga, you go to Lake Valor or any other lake, you surf to a cavern in the middle and in there you will find Mesprit.
they are the three legendary trio of lakes they live in three lakes azelf in lake valor mesprit in lake verity and uxie in lake acuity
the 3 lake guardians are uxie and azelf and mesprit.
it's in lake verity
your home lake
Mesprit, azelf, and uxie
The Lake Guardians
mesprit,azelf,uxie
You'll find The Lake Trio: Mesprit, Uxie and Azelf.
After you catch the main legendary (dialga, palkia, giratina) go to lake verity and mesprit will be in the cave in the middle of the lake. Talk to it and then it will flee and appear on your map app.
in each of the Pokemon lakes, after you have visited the spear pillar and found giratina go to the centre of each lake to catch mesprit, azelf and uxie. (Mesprit will wonder so you have to follow him)...^_^