Some players say the only Reaper on the Amissa map try to spawn at the end of the AB Cave. But it can't.. :( Unfortunately it's a bug.. I didn't see yet...
You can check the concrete cave here: Fan Made Amissa MAP Rev.2 (Discussions tab),
or here: https://i.ibb.co/9bkZsZQ/ark-amissa-map.jpg
And try to find the beast. Good Luck:)

you can force summon a reaper by geting namless up in dead forest angry enhough that it cant kill u he will rorar for reimforcements reaper kings