|Seven-headed Hydra by David C. Sutherland III from page 29 of the Blue Book|
While working on the Monster Reference Table for Holmes Basic, I was reminded that Hydras in these rules are given Hit Dice that "varies with number of heads". In the description, Holmes then notes that "Each head is represented by one hit die of 6 points, so a three headed hydra has 18 points, a 6 headed one, 36". This is more open-ended than either the original Hydra entry in Monsters & Treasure, or the versions in the Monster Manual or the B/X Expert Set, which each limit Hydras to having 5-12 heads. Since Holmes places no limitation on the number of heads, and gives the specific examples of 3- and 6-headed Hydras, we can extrapolate the following table:
We can also postulate that a young Hydra starts with 1 head/1 HD, and add heads and HD as it ages (the above table can also be used for rolling a random Hydra with a d12).
This is one small way that Holmes made the original OD&D monster list more appropriate for Basic levels. I think it's often overlooked because (1) the illustration shows a seven-headed hydra (see above) and (2) everyone is more familiar with the 5-12 HD Hydra. To emphasize this feature I placed Hydras right after the 3HD monsters on the Monster Reference Table, making them entry 3-12. They could appear as a 3rd level monster if you use the table for Wandering Monsters as I previously suggested.
A few other notes:
-There's a silhouette of a body lying below the Hydra in the picture above. Presumably there was another fighter on the ledge who was taken out by the Hydra, which still has all 7 heads in action. Doesn't bode well for the sole remaining warrior.
-The 1st-3rd printings of the Blue Book contained an extra paragraph at the end of the entry for Giants that mentions Hydras among their "pets".
The reference to Hydras as guards for Giants is from Monsters & Treasure, page 9. This paragraph (along with the entry for Nixies that I posted yesterday) was deleted from the second edition (Nov '78) of the Blue Book. I don't see any reference to Hydras in the descriptions of the various Giants in the Monster Manual.