Major spoilers ahead, so you have been  warned. If you want a hint on how to unlock him other than "Open the Chest", look at part 1 just under this.

Part 1, The Hint:

Find the vault. Find something crystalline, alive, and red. Find where the interdimensional travelers came from. Find a room guarded by something with immense power. Narrowly escape death into a strange place by giving up radiation.

Under this is exactly how to get Beetle

Part 2, Finding the Crown Vault

The crown vault can be found under the same conditions as the proto chest. Get a crown, come back to the temple thing and there should be a teleporter that takes you to the crown vault. On the floor, there are 4 places to put artifacts that are scattered throughout the wastelands. Progress for putting the artifacts in the vault is saved between runs. You can only visit the crown place 3 times a run.

Part 3, The First Artifact

Enter the crown place with 1 health the first time you go to the crown place. The artifact should be there. Place it in the room upon coming back to the crown place. All the artifacts are items, so it limits your guns to 1 while carrying them.

Part 4, The Guarded Artifact

There is a room behind the throne in 7-3. Use Crystal's Throne Butt to teleport behind it and get the artifact and then loop to be able to put it in the vault.

Part 5, The Stolen Artifact

Go to the IDPD HQ (Mash your interact button on an IDPD van while it is unloading troops) and get to the captain boss fight arena. Before fighting the captain, go to the left and there is a hidden elevator. Grab the artifact and finish the fight. Don't kill the captain, take the guardian pet to escape and continue your run. Place the artifact in the vault at the next opportunity.

Part 6, The Lost Artifact

Get to the Corrupted Caves (The red world thing) by running into a red crystal heart. DO NOT KILL THE HEART, RUN INTO IT. Also, there are white hearts, kill those. They cause more trouble if you touch them. There is a secret room that leads to a room with the Twins pet. There is another secret room through another secret wall corridor above it containing the artifact. take it and put it in the vault. A red crystal heart is guaranteed to spawn in a looped run.

Part 7, The Fight

Upon putting all of the artifacts in their spots, they spawn enemies from where they were taken from. (Radiation Boys from world 7, Red Crystal Boys from the Corrupted Caves, IDPD from the IDPD HQ, and Normal Boys). You have to fight 2 of the factions, the other two factions, and then all 4 at the same time before the artifacts make a big key you can unlock the chest with. You get beetle and a big boy laser gun that you should immediately put in the proto chest.

Hope this helped! :)