A mantis piercing is just an ordinary nostril piercing, well two, but facing forwards at the tip of the nose:
You can do this with a single bar across the nose, but this has a higher chance of coming out at some point.
Mine are two separate piercings that go into the nostril. The visible effect is the same and it’s easier to change the jewellery.
It’s an unusual piercing. When I had it done, my piercer said she had really enjoyed doing it. I’m not sure she was so delighted about having to stick her fingers up my nose.