They can appear the same ways as they where introduced:
10 to 99 soul summons where at 99 souls it's the highest chance with 1%

And a new way:
Whenever a hoarder chest, the loot chest that costs basic to omega essence to open, is generated (not to mistake with opening one) a tameable boss spawns together with it

The second way is the only way to aquire the unique versions of the tameable bosses

Also the darts were removed Cuse they were the worst taming ammo in ark omega and Noone with a bit of knowledge in the contend of the mod would used them...

The best ammo for torp, including raid torp, is the normal arrow on a unique/legendary warp bow