the druid's animal companion( AC ), has become the focus of alot of my concerns lately.
i have found by reading many, many ....posts that the AC falls somewhere between just a normal animal and magical beast.
it is ALMOST, but not quite a group to itself.
by linking with a druid the animal becomes better than it would be in the wild.
Now, as with your question about if it would revert to its normal stats in the beastiary if one was to let it go. it would depend on if one was to give the AC an int boost. any animal creature with a int of more than 2 is not an animal anymore.
i dont know if the stat gains are akin to a SU ability through the linking with a druid or a permanent stat gain.
if its perm, then the AC, once released is a magical beast, if its a gain through the link, then it reverts to a normal creature of its type.
i hope this is of some help.