Hi so for tonight's session I am making a mini boss 10 lvl druid. I want him to have a rhino as an animal companion and I believe that if he awaits that a battle is at hand, he would have buffed out the rhino. I want the animal to be the bruiser, as my druid will be mainly a caster in animal form. So my basic question is: if I have a rhino and I cast on it animal growth and greater magic fang how will his attacks be modified? I understand that he will have a gore attack for +18 (2d8 + 17). Is that right? Also, would his powerful charge ability dmg will change because of size change? And last question (for overkill). Due to the share spells link that a druid has with his companion, would the same buffs apply to a druid shapechanged in an animal while casting them? (Overkill in my opinion). Thanks
Powerful Charge is dependant on the creature size, the damage values given in the MM3 are different than those of a Rhinoceros however (it's just to show the size dependency). I would expect that since the base damage of the rhino's powerful charge is 2x his regular gore (2d6 versus 4d6) that his powerful charge remains x4 after his size got increased via animal growth. A 2d6 becomes a 3d6 however and not 2d8 (with improved natural attack).
The base rhino has a +13 to hit for 2d6+12 to hit, applying buffs in order gives:
Animal Growth (+8 str, +4 con, +2 NA, -2 dex, DR 10/magic, +4 resist) gives the rhino a +16 to hit (+4, -1) for 3d6+16 [Implying 6d6 + 32 for Powerful Charge]
Greater Magic Fang (+2 to hit/damage, you are caster level 10 making it +2) enhances the rhino to a +18 to hit (+2) for 3d6+18 [Implying 6d6 + 36 for Powerful Charge]
I can also suggest buffing your animal with barkskin if you want to make him truly 'evil', the rhino is already a player killer however since not many classes can survive getting hit by said charge.
All spells cast on yourself can also affect your animal companion (assuming that the companion is within 5 foot), the companion loses the benefit of the spell(s) if he goes further away from you and will not regain the benefit (literal quote of PHB). The RAW can not be turned around so that spells that are cast on your companion also effect you so the answer to that would simply be 'No'.
I would say that the share spell ability is only useful if you use your animal companion as a mount myself.