[RPG] How does a Gnome Battle Smith artificer (UA) riding his iron defender control it

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I don't see any reason a gnome Battle Smith artificer (from the May 2019 UA) couldn't ride his iron defender RAW–it's got the appropriate anatomy and is one size larger.

But how is it "Controlled"? Controlled has a specific meaning in mounted combat: A controlled mount acts on the riders turn and does not get attacks, an uncontrolled mount goes on it's own initiative and gets attacks. An intelligent mount can only be uncontrolled, with an animal intelligence mount you get the choice.

I don't believe the Iron defender has a free will though, it is always controlled by the Artificer that constructed it.

So if the Iron defender was being ridden, I'm guessing it could be in "Uncontrolled" mode, going after the Artificer, and still get it's regular attack actions–but directed by the Artificer (Still essentially controlled).

On the other hand–you might want to be in "Controlled" mode if you didn't need the Iron Defender's attack–for instance if you were going to charge with a lance, have the artificer hit, then run away.

So my questions are–is my research (and assumptions) correct as I stated above, and if so, is there any way to switch between controlled and uncontrolled mode?

If you were riding an animal, I'm guessing the "Mode" might be determined by how you control it… or perhaps it cannot act on it's own if it has a bit in it's mouth and reigns? For the Iron Defender I can't think of any differentiation that would apply except the Artificer's will.

It is up to you and your DM, but likely it would be controlled.

Iron Defender (UA 11-12):

By 3rd level, the tinkering in your free time has borne you a faithful companion, an iron defender. This metallic creature resembles a hound, a cougar, a bear, or another four-legged creature of your choice. It is friendly to you and your companions, and it obeys your commands.

The Iron Defender has an INT of 4. The Dragon Talk: Sage Advice on Mounted Combat admits that this is vaguely defined, so rule at your discretion. According to Jeremy Crawford, most players make the choices for their mount if it isn't an important NPC-tool of the DM.

Mounted Combat and Controlling a Mount (PHB 198):

A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount

While you're mounted, you have two options. You can either control the mount or allow it to act independently. Intelligent creatures, such as dragons, act independently.

The anatomy is suitable as it resembles a four-legged creature of your choice (Gnomes are size Small see PHB 37, Iron Defenders are size medium, see UA 11) and the creature is willing (see first quote).

I would rule that the Iron Defender is not an independent creature because it already moves on your initiative and enacts most of the benefits of a controlled mount:

In combat, the iron defender shares your initiative count, but it takes its turn immediately after yours. It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take one of the actions in its stat block or the Dash, Disengage, or Help action.

This is congruent with what a controlled mount does.

The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it.