Outside the Warcraft universe, the incubus are the male equivalent to the succubus. It is possible that it's the same way in the Warcraft universe, and therefore perhaps they are the males of the sayaad race. Such context was not given in The Last Guardian.
It should also be noted that both succubi and incubi are powerful 'sensual' demons and may not take gender into account, so a summoned incubus may become infatuated with/vis versa their warlock or summoner regardless of gender. The succubi are well known to do so- with much jealousy.
On 4/14/2011, an official member of Blizzard said that they would be making male/female counterparts to all Warlock minions (eventually) in the game. This would suggest the male counterpart to succubus to be incubus and not 'male succubus':
I'm happy to share we actually do have plans for offering players a choice between male and female versions of all warlock demons. This is a task already in our system, though I don't have a time frame for you at the moment. Other art tasks could always take precedence, but we're committed to making this happen. :) - Zarhym, Blizzard Community Manager
Q: Do incubi exist?
A: There are several different rumors concerning the male counterparts to the demonic succubus race, and it's clear that the succubi are responsible for all of them. A few of the more common rumors are:
- Yes, there are incubi, but the spell to summon them has been conveniently forgotten by mortal practitioners and Burning Legion agents.
- Incubi are kept as slaves on their home planet, having been rendered incapable of escape or independent movement.
- The succubi consumed the males of their race when they were brought into the Burning Legion. (Alternatively, the act of devouring the males is what caught the attention of the Burning Legion.)
The Last Guardian
Published on 10/26/02, The Last Guardian follows the protagonist, Medivh, and his inner and outer struggles against the Burning Legion. Only stated once in the novel (was this quote) including the word 'incubus'.
"While you have been fighting only a few, my brothers in sorcery have brought in others. A legion of others. Every incubus and petty demon, every nightmare and shadow-hound, every dark lord and captain of the Burning Legion. All have come here while you have fought these few." -The Last Guardian, by Jeff Grubb.
This leaves wide speculation that a literal incubus was being referred to, and was the only "lore" stating an incubus existed in Azeroth or Warcraft world until a Blizzard employee announced male/female counterpart demons. However, in literature, incubus can also be used as a descriptive and figurative term (Incubus also can mean nightmare- which in this case would refer to horrifying or scary demons- and can be used as an adjective for anxiety or wretchedness). This leaves some question as to which form of "incubus" Jeff Grubb was implying.