I like it, but ethereal is not needed imo. The spell is interesting though, because with the nature school... it is difficult with the usual johktari lineup to get more than 4 base dice with it other than via a bear. Care would be needed with the wording to make sure that any melee+ bonuses on the creature are not included in X, and it would almost certainly be friendly animal.
Going on that criteria, without any effect at all (or even along the lines of 8+ daze) the spell would probably cost about 5 mana. A good effect would raise it to 6. Conservatively, I'd say 6 with a good but not stellar effect to allow expansion of the animals past it's current limit of 5. It's interesting in that it properly rewards the beastmaster for playing thematically.
All in all, minor critiques from me... but I love the card.
Glad you love the idea. I thought it seemed pretty flavorful!
As far as Ethereal, the spell doesn't *need* it, but this game is all about flavor. Almost everything incorporeal is a spirit of some kind, so it would make sense to give the spell Ethereal because spirits should fight spirits just fine! (FWIW Every incorporeal creature printed so far has Ethereal.)
I was intentionally leaving the casting cost as an X. From what I've seen, about 1 EV damage = 1 mana cost. (Hurl Rock is aberrant.) So, with just Ethereal, I bet the cost could just be straight up X, where X is the damage it deals. (Since it makes the player work slightly harder to have an animal.)
Agree that the target should probably be Nature creature and not non-Mage creature.