On the battlefield (no longer: in play [C.R. 403.5]), Primordial Hydra's converted mana cost is 2. When finding an object's converted mana cost, the X in its mana cost is 0 except while it's on the stack (that is, the X is zero even while it's on the battlefield), and is the value chosen for X while it's on the stack (C.R. 202.3e).
EDIT (Aug. 8, 2018): One rule was renumbered with Commander 2018.
EDIT (Feb. 17, 2021): Clarification.
On the battlefield (no longer: in play [C.R. 403.5]), Primordial Hydra's converted mana cost is 2. When finding an object's converted mana cost, the X in its mana cost is 0 except while it's on the stack (that is, including while it's on the battlefield), and is the value chosen for X while it's on the stack (C.R. 202.3d).
Shu Yun, the Silent Tempest WUR Voltron Control
Temmet, Vizier of Naktamun WU Unblockable Mirror Trickery
Ra's al Ghul (Sidar Kondo) and Face-Down Ninjas
Brudiclad, Token Engineer
Vaevictis (VV2) the Dire Lantern
Rona, Disciple of Gix
Tiana the Auror
Hallar
Ulrich the Politician
Zur the Rebel
Scorpion, Locust, Scarab, Egyptian Gods
O-Kagachi, Mathas, Mairsil
"Non-Tribal" Tribal Generals, Eggs
EDIT (Aug. 8, 2018): One rule was renumbered with Commander 2018.
EDIT (Feb. 17, 2021): Clarification.
Thank you!
Shu Yun, the Silent Tempest WUR Voltron Control
Temmet, Vizier of Naktamun WU Unblockable Mirror Trickery
Ra's al Ghul (Sidar Kondo) and Face-Down Ninjas
Brudiclad, Token Engineer
Vaevictis (VV2) the Dire Lantern
Rona, Disciple of Gix
Tiana the Auror
Hallar
Ulrich the Politician
Zur the Rebel
Scorpion, Locust, Scarab, Egyptian Gods
O-Kagachi, Mathas, Mairsil
"Non-Tribal" Tribal Generals, Eggs