Nibblegar, Sheartooth, and Megalobite can all be found in the wild. So, if you didn't choose one as your starter, you'll have the chance to add it to your team. Albeit, near the end of the story.
Nibblegar will evovle into Sheartooth at level 17 and then into Megalobite at level 35.
Nibblegar and it's evolutionary line are found in the waters of Ixqun. To access the specific location where they are located, you'll need to use your surfboard. It's located directly north of the transporter as pictured below.
Once, you have arrived to the spawn location save your game. Then, spray a Potent Scent. To ensure you get the most out of your scent, only save before you use it. This way you'll simply be able to reset and start again without using another if you don't find what you are looking for.
Select the fishing pole icon and start hunting! Evenutally you will come across either a Nibblegar or one of it's evolutions. It doesn't matter which one you catch as you'll be able to reset the potential points at a trainer hub to allocate as you desire.
Now that you have Megalobite, here is a great moveset that will earn you some wins in Coromon PVP.
This set allows Megalobite to function as a mixed bulky attacker. You have your obligatory STAB (same type attack bonus) move in Bubbleblower. In addition, Quicksand allows you to hit Electric types that would otherwise laugh at your Megalobite.
Crunch will hit other water types hard and Headcrash destroys Ice types. Be aware that the latter will cause Megalobite to lose 30% of the damage it outputs. This hinders his survivorability a bit, so be careful not to spam it too much.
The best trait for this build is Fully Rested. This ability allows your Megalobite to reduce damage taken by 40% when it is at full health.