For some reason, I didn't have any aliens. Also, I didn't see tanks or airplanes in the original game or in Hammers of Fate.

I also didn't see the airplane – I didn't play through the Hammers of Fate campaign, but I did find the tank in the original game for the human faction.
And the alien is a bit awkward – his summoned creatures appear. Once, I didn't think it through and assembled a powerful army of dwarves (40+ dragons) for the alien in mission 4, and then summoned rakshasas. The entire army just vanished. But his stats are amazing – at level 26, he has 24 in magic and attack, and around 20 in defense and knowledge ("Mentorship" skill).
As for his appearance, maybe it depends on something. For example, in both cases, I switched from another hero to Zehir on the 1st day of the week after the message "Are you sure you want to...?" appeared. Maybe that's why?