Take the "Animal" boost spells. Bull's Endurance, Bear's Strength, and so forth. Make a new spell called "Animal Essence". Animal Essence Functions much like Resist Energy does, allowing you to choose which of the different Animal Boost spells you would like to make use of much as you can choose which Resistance you would like to cast.
Create a "Mass Animal Essence" at higher levels.
Classes that are restricted in which Animal Boost they can cast are not given Animal Essence and are still restricted as such.