I've been iffy about the animal companions of rangers ever since 3.0 (which is when i started DnD), the animal companion of a druid is all nice an fine, but when it comes to rangers I don't see much of a point to having an animal companion, since such are generally a bit weaker than a player character, but the ranger's companion will always be on half the party's level, making it more and more useless as the party advances. In a 20th level party the ranger could have something between for example a dire rat with +6 HD (that would give it +4 BAB I think) +6 Natural armor, +3 Str and a few nifty bonus tricks and specials, or how about a polar bear with close to no bonuses.
Now having a polar bear by your side sounds pretty awesome but considering that a player character of your level could alone take on a young adult dragon, that's suddenly not that much anymore.
In the end of it all the ranger's animal companion will just deal minimal amounts of damage (if it hits at all!) and die pretty quickly due to way to low armor and Hit points.
The only thing I see it useful for would be flanking, since the enemy will most likely ignore that little critter scratching the polish of their armor anyway.
I know a ranger's companion shouldn't be as powerful as that of a druid, but it should at least be able to do something.
So I would suggest changing the Rangers animal companion level from Half his ranger level to his ranger level-3. That would still keep the animal significantly less powerful than anyone else but it would at least not be almost entirely useless.
Thoughts?
Okay I have by now realised that there is already a thread on exactly this. So, I guess this thread is a little superfluous. :(