This is kind of growing beyond a quick question, but... You're asking too much of an animal companion. The druid & ranger abilities are (Ex), meaning completely non-magical. You are basically such an amazing outdoorsman, you find and train an animal and help make it extraordinary. The class feature says this: "A 1st-level druid’s companion is completely typical for its kind except as noted below." And noted below is what your companion gets, like more STR & DEX, extra tricks, etc. It does not gain the ability to speak, it does not gain an non-neutral alignment or the ability to devote itself to a high moral cause. It is just an animal that you have taught to be really amazing.
Now compare the paladin special mount. It is a (Sp) ability, already showing it is magical. The ability states: "Upon reaching 5th level, a paladin gains the service of an unusually intelligent, strong, and loyal steed to serve her in her crusade against evil." Then looking down at the chart below, it specifically shows INT increases. Also, it further states this mount is no longer an [Animal], but is now a [Magical Beast].
A familiar is somewhat similar. You have to perform a magical ritual to obtain one, they are magical beasts, and they are specifically indicated as gaining INT increases.