Playing a human artificer battle smith with the spell Enlarge/Reduce, I want to use the Steel Defender as a mount. It is a medium size construct that becomes large size under the effects of the spell.
You may design the Steel Defender as you like, given that:
You determine the creatures appearance and whether it has two legs or four; your choice has no effect on its game statistics. (Eberron – Rising From the Last War, pg.61)
Even though it has “no effect on game statistics,” we can assume we care enough to design it in such a way it may act as a suitable mount.
The PHB rules for a mount in combat state, on pg.198:
…and it has only three action options: Dash, Disengage, and Dodge.
But also says, shortly after:
An independent mount retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes. (emphasis mine)
The Steel Defender has an intelligence of 4, meaning it does not automatically (and always) act as an independent mount (it is not sufficiently intelligent). Yet, according to this question and accepted/upvoted answer all mounts may act independently.
Yet in the rules-text for the Steel Defender (pg.61 of Eberron – Rising From the Last War) it says:
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…
One more relevant rule:
…the Steel Defender shares your initiative count, but it takes its turn directly after yours.
Which is the same as the PHB rules for a mounted creature.
If I use my bonus action on my turn to command it while mounted, may it still use its Force-Empowered Rend or Repair action on its turn?
What are the benefits of having a mount?
How does a Gnome Battle Smith artificer (UA) riding his iron defender control it?
How do I determine if a creature is too intelligent to be a controlled mount?
Does a controlled mount share its rider’s turn?
Can a controlled mount make an Opportunity Attack?
Essentially what I’m going for (Image from Final Fantasy Brave Exvius):
Not that the rules care what it looks like. It could just as easily be a mechanical horse.