First make sure that the mantis is free from other animals. Take care of the mantis. Hug with a dragon or another big animal that does not attack the mantis. And then you can calmly give him the deadworm horns
On Fjordur, you can find rocks in the volcano area that resemble fossilized Death Worms, which can be harvested for Death Worm Horns, making taming them much easier there, if you have the rest of the necessary equipment
Please don’t listen to the boy who said with bug repellant, ghille and cactus broth that the aggro range is “literally zero” cause it’s not.. you can get super duper close without aggravating it but you can’t get too close so still be careful and try not to stand so close you can touch it or it will STILL AGGRO.