The Incubus model is a new feature coming to Warlocks in patch 9.2 Eternity's End that offers a male version of the Succubus pet.
Initially, everyone assumed the new model would make its way to the game as a new Glyph but now the PTR is live, three items have been data-mined that show how the new Incubus model will be summoned.
The first option is the Glyph of the Tormentor, a new Glyph that will make the Summon Succubus spell spawn either at random. It is not known where the pattern will come from yet, but it likely won't be difficult to obtain and will find its way to the auction house shortly after release.
How To Pick An Incubus Pet As A Warlock In Eternity's End
Additionally, there seem to be 2 new items that will guarantee the spawn of the one the player prefers.
The Barbed Collar of the Succubus, and the Barbed Collar of the Incubus, each say "Strengthen your pact with you Succubus/Incubus ensuring that your ritual will only summon them.
These items aren't Directly in the game at the moment, so it is still unclear how they are obtained, but having the option to select the pet you want or have it random each summons is a nice feature added for players, as long as the method to obtain them is not long.
We will know more once the PTR receives more patches and we get closer to the release of Eternity's End.