When an invisibility to animals spell is cast, the creature touched becomes totally undetectable by normal animals with Intelligences under 6. Normal animals includes giant-sized varieties, but it excludes any with magical abilities or powers. The enchanted individual is able to walk among such animals or pass through them as if he did not exist.
For example, this individual could stand before the hungriest of lions or a tyrannosaurus rex and not be molested or even noticed. However, a nightmare, hell hound, or winter wolf would certainly be aware of the individual. For every level the caster has achieved, one creature can be rendered invisible. Any recipient attacking while this spell is in effect ends the spell immediately (for himself only).
The material component of this spell is holly rubbed over the recipient.