http://dndtools.pw/spells/spell-compendium--86/phantom-stag--4628/
I recall having issues with this spell when I used it, in that it doesn't provide nearly enough information about the stag.
Strength and BAB: No clue. The d6+9 damage presumes a Str of 22 (+6; x1.5 damage for a single natural weapon), but only if its not considered a magic weapon (it later gains properties, but it never mentions it being a +1 or better weapon, either). I guess Str 22 and BAB +5 to reach the +10 attack bonus w/ -1 size included and no enhancement bonus....
Dexterity 20, based on AC note
Con and HD: No real way to tell, but I think Con 10 and HD equal to CL -1 (thus, it's getting 5 hp per HD) is reasonable. It does not gain BAB from these HD since its attack bonus never increases; it's just for resolving HD-dependent effects.
Mental ability scores: Absolutely NO IDEA how to figure these out. Hopefully, it's never needed.
Saving Throws: So much for that... I have no idea here, either. Dex adds 5 to reflex, Con adds 0 to fortitude, Wis is a wild guess. Base saves...going to have to figure something out.
Immunities and qualities: It's a corporeal creature. Despite the spell name, it is not a shadow spell. Is it subject to ability damage? Maybe. Critical hits? Probably. Mind-affecting? No idea...if it were you could dominate it into allowing a new rider which I'm not sure should happen, but I see no reason for it to be totally immune to enchantments, either. Is it an animal? Magical Beast?