Under the under growth of the smelly poo . If you don't believe it's real then ...............
Chat with our AI personalities
jiroxy ball...
After you beat the elite four and the galactic biulding
You have two chances to catch Mesprit. If you accidentally make it faint both times, you can't catch it anymore. If you made Mesprit faint once, find it, trap it, lower its hp to 1 using false swipe (you should have gotten the TM), let it escape, then encounter it again and use a quick ball.
A Master Ball
You catch Mesprit in HeartGold in Route 48 on either Mondays and/or Wednesdays while listening to DJ Ben's Marching Band. Or at least I think! Hope I helped^_^ ~ Kaede Nagase.