Just what falls under the umbrella of Animal Friendship?

Animal Friendship(Enchantment/Charm)

Sphere: Animal
Range: 10 yds.	Components: V, S, M
Duration: Permanent	Casting Time: 1 hr.
Area of Effect: 1 animal	Saving Throw: Neg.

By means of this spell, the caster is able to show any animal of animal intelligence to semi-intelligence (i.e., Intelligence 1-4) that he desires friendship. If the animal does not roll a successful saving throw vs. spell immediately when the spell is begun, it stands quietly while the caster finishes the spell. Thereafter, it follows the caster about. The spell functions only if the caster actually wishes to be the animal's friend. If the caster has ulterior motives, the animal always senses them (for example, the caster intends to eat the animal, send it ahead to set off traps, etc.).The caster can teach the befriended animal three specific tricks or tasks for each point of Intelligence it possesses. Typical tasks are those taught to a dog or similar pet (i.e., they cannot be complex). Training for each such trick must be done over a period of one week, and all must be done within three months of acquiring the creature. During the three-month period, the animal will not harm the caster, but if the creature is left alone for more than a week, it will revert to its natural state and act accordingly.The caster can use this spell to attract up to 2 Hit Dice of animal(s) per experience level he possesses. This is also the maximum total Hit Dice of the animals that can be attracted and trained at one time: no more than twice the caster's experience level. Only unaligned animals can be attracted, befriended, and trained.The material components of this spell are the caster's holy symbol and a piece of food liked by the animal.

This is an excerpt from the AD&D 2e PHB, namely the Animal Friendship spell. It's come up several times that the description of the spell doesn't specify what kinds of animals it affects (actually, it does: ANY animal). Sometimes in AD&D "animals" mean anything of a certain intelligence level. Sometimes it means any creature that naturally appear on Earth (including giant variations). Sometimes it means something more broad.

The question came up on saturday with the Hounds; should Heimir, the God of Hospitality, grant his priests a spell which could be used on drakes? The Hounds, after killing the drakes near Arodariath, took home a large clutch of eggs that were still alive. Warming one for several days over a slow fire, Myndil the Merry attempted to use this spell on the hatchling.

It failed. Why? The spell as written should function. Yet, Heimir canonically has no power over dragons or dragon-kin. Thus, while the spell might work on sylvan creatures or those whose domain is traditionally a household (even if they are magical in nature), it had no ability to affect the drakes.

My players agreed that this was a reasonable ruling. It's all part of that DIY, Build it Yourself, Customize it at your Table atmosphere.