SO as an example I have a beastmaster ranger and an artificer with the battle defender as part of their class features. Are these creatures allowed to attune to magic items, and are there any caveats I'm missing here?  I cant see a clear answer in any of the rules.

The rules on Attunement allow for any creature to attune to 3 magical items.

Specific class abilities which grant pets/companions might specify more specific rules about that particular pet/companion.

But note that attunement requires: "This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity." So if the creature isn't intelligent enough to know how to "practise" with the magical item, then perhaps they won't be able to attune to it.

Quote from Farling >>

The rules on Attunement allow for any creature to attune to 3 magical items.

Specific class abilities which grant pets/companions might specify more specific rules about that particular pet/companion.

But note that attunement requires: "This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity." So if the creature isn't intelligent enough to know how to "practise" with the magical item, then perhaps they won't be able to attune to it.

  This is a good point, and for me it comes down to this: can a creature follow your commands and respond? Then: Does it have the ability to cast?

If the answer is yes, it can attune to magical items that don't grant casting abilities (so it can attune to, say, a Brooch of Shielding but not a Necklace of Fireballs. I know this goes against what PCs can do, but).

If the answer to both is yes, well.... have fun with your new tank-mage! :D

Enjoy your Ape with a Cloak of Protection, Half Plate Barding, Eldritch Claw Tattoo, and a Belt of Giant Strength XD

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As a general rule, I say anything with INT of 3 or more (I believe 3 and below is where most mind magic ceases to work) can be instructed to attune to items. For reference, Familiars such as Sprites, Imps... Octopus?... all have INT that qualify.

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And that's to say nothing if you find/have a Druid that casts Awaken on a beast and it becomes your friend! With an INT of 10 it can absolutely attune to things, no problem. And since it can read/speak/understand Common now, you can give it explicit commands/guidance on how to do so. :D

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(Yes, everyone, a Staff of the Woodlands can totally break a game quicker than you can say "why is there an elephant here?")

My dog is smart enough to have favorite toys, plays with different toys in different ways, has certain behaviors that he repeats with items he likes more than others, knows what I expect him to do when I put a leash on him or throw a frisbee, etc... I don't think that a DM needs to go out of their way to say that animals (especially trained or psychically controlled animals) can't attune just because they're dummies. Using/wearing magic isn't rocket science. 

These are all good responses. It seems that the general consensus is that in general, pretty much if it has some sort of thought process, they can attune. Just seems weird for a class granted ability to be further enhanced by magic.

Attunement implies some higher level of connection to the magic item than just using the item; so whilst a dog could be use an item that didn't require attunement, I don't think it would be capable of making the "higher level of connection" required for attunable magic items.

That was my general thought process as well, in addition to the class granted ones....I don't think they were designed with that in mind.

I think "yes, within reason," is the best answer.

If the DM thinks the pet can use the item, then it can attune it. I can't imagine a dog picking off beads from a necklace of fireballs, but it can scratch with a magic claw on its paw or gain the passive benefits of a cloak.