Since when did the mob summon players?
 

As a result of searching, there was such a function before June 11 1999.

wiki.project1999.com/Patch_Notes
High-level non-player characters with the ability to summon characters can only use the summoning ability if the characters cause more than 10% damage to the NPC.

I couldn't find it in the patch note before this fix.
Then, did summon exist from launch date(March 16, 1999)?

But summon is kind of cheap trick to prevent easy farming of High-level(51+) mobs...

Don't you remember anyone?

http://www.project1999.com/forums/sh...ad.php?t=16165
Nilbog: All level 51+ npcs have gained the ability to summon.

In p1999, summon was added after plane of sky.

Honestly rather than rooting mobs dragons they should cast uninteruptable gates and summon Players in ntov

EDIT: Misread OP

He's asking about summoning, not gating.

And I know vox and nagafen had summon either at launch or very, very early. it was certainly in by the time PoF launced.

Oops; fixed post, thanks for the catch.

Venril Sathir summoned me multiples times once when I got lost and somehow aggro'd him.. I somehow got out alive, to my amazement. I remember being so fucking scared, and also making a lot of people angry when I inadvertently parked him at the zone-line and ruined the entire zone for dozens of players.