Once I tame a mantis what do i need to feed it?

Solution 1:

Based on a quick Google search, it states that:

The Mantis is unable to be tamed through the standard tranquilize-and-feed methods, and must be tamed non-violently.

To feed it, place a Deathworm Horn in the last slot of your hotbar, then when approached press "use" (default E) to feed it. You cannot feed them all the horns required for taming at one time, you must wait a few minutes after each feeding to give it the next one, and repeat until the taming meter is filled.

The Mantis is normally aggressive, so Bug Repellent is needed in order to get close enough to feed it. A set of Ghillie Armor is also useful, but not required. The Mantis becomes aggressive if you are too close. The taming progress will become halted until it loses interest in whatever it was fighting, or even decrease if it's aggressive for too long.

A good trap to tame the mantis is a single foundation (Stone, Adobe, or Metal) with 4 walls on the first level, and 3 walls on the second level.1 Lastly add two ramps to the last opening on the second level. Part of the mantis will stick outside of the trap and you can feed it. You will not need any Bug Repellent or Ghillie Armor with this method.

And that it's a carnivore, so meats are the diet

Solution 2:

Manti, like the Pulminoscorpius, eats rotten meat once it is tamed.