Easiest way to raise a Reaper:

Create a pen big enough for a Maewing.

Stock the maewing with cooked meat.

Birth the reaper (easier if you have pheromones so it doesn't attack) and go to your Maewing and use "pickup nearby babies".

This will put the baby in the Maewing saddle and now it cannot roam around OR attack you.

More Reaper Taming & KO Tips