How to deal with a Manta's aggressive behaviour:

Step 1. Craft a gun from a Shipyard (that other thing you construct in the shipyard category.

Step 2. Make sure you have plenty of bullets, if not, craft some from the crafting tab.

Step 3. Make a ship that is entirely (or mostly) square, place small tiles around it to prevent the Manta from flying in.

Step 4. When a Manta gets stuck in the ship, shoot it, it'll either fly away or die.

Step 5. You will possibly anger the Manta's companions, be prepared to take them down.

Step 6. If the Manta do break your ship, be prepared to move around at all times, they'll swoop in for the kill.

Step 7. Once the Manta have calmed down, continue to shoot them down and aggrite them.

Step 8. Claim the meat from the corpses.

Step 9. Cook the food near a campfire.

Step 10. Eat up.