They spawn on centre and scorched earth. You find it in the wild at night and kill it. Once killed they drop an alter, you place the alter at night it will ask for a tribute (those used for boss fights).

If you have the tribute for example thyla claw it will then spawn again to which you kill it and it will ask for another tribute this can range from sarco skin to giga heart and even alpha tributes such as alpha T-Rex tooth.

If you do not have the tribute it will spawn after a 5 minute time but this will cause damage to the alter. You have 3 failures allowed and the 4th will destroy the alter and the dreadmare will be gone.

Hope this helps

Smokey