To tame a mantis one can follow all sorts of advice. Or one can approach them as one would a shadowmane. Build a 2x2 box with 2 walls high of stone and a single doorframe in there somewhere and 2 ramps (one wide is enough) and lead your prey inside. Replace the single doorframe with another wall. Get out of render distance to unaggro. Remove ramp to keep your specimen safer from predators. Feed it through the wall, you can even approach it's head. (I am tired of wasting the horns I chased down so there)

Note that getting them into a cage is significantly easier if you can choose a terrain, however this baby requires not merely an argy but a wyvern to carry. To get a wyvern, on Fjordur, get a Desmodus, wait til nighttime, sneak into nest and use realm teleportation. Nab egg as the black screen hits. Bring a fjordhawk just in case.