I pushed the envelope a little for my Battle Smith concept, but they're an NPC in my game so I get to do that. :)

The backstory is the character suffered a horrible injury as a child that left them essentially immobile. They overcame this by tinkering incessantly with mechanisms to regain their mobility, and this ultimately is how they became an Artificer. By 3rd level, the prosthetics now have an independent mind.

In game terms, this functions exactly like a mount, except it's a "permanent" mount. I changed three thing to make this work: allow the Medium character to "ride" their Medium mount; if they dismount or the Defender falls to 0 HP, the character is considered prone; when "mounted", the character can move the Defender on the character's turn, rather than on the Defender's turn. In all other ways, it works exactly like a mount.

Overall, this is likely a decrease in versatility compared to a Small character riding a Steel Defender as an actual mount, but I think it's worth it for the flavor.

My Steel Defender is a three-necked/headed cassowary that my Gnome artificer rides into battle. The three necks/heads allow her to pick up things without adding arms.

Quote from Mergon >>

One of my favorite steel defenders to date, is a Terminator (ala. Arny) minus the flesh. Still trying to come up with a way to give it a body job (maybe a Manual of Flesh Golems if I can find ione). :)

 give him a spell storing item with disguise self in it?

Quote from Ziggy12 >>

Quote from Mergon >>

One of my favorite steel defenders to date, is a Terminator (ala. Arny) minus the flesh. Still trying to come up with a way to give it a body job (maybe a Manual of Flesh Golems if I can find ione). :)

 give him a spell storing item with disguise self in it?

 Despite the name, nothing in the Steel Defender's description states that it needs to actually be a steel creature. It could just as easily be made of stone, clay, or some kind of synthetic flesh if that's what your DM allows.

Quote from Luxxan >>

I pushed the envelope a little for my Artificer concept, but they're an NPC in my game so I get to do that. :)

The backstory is the character suffered a horrible injury as a child that left them essentially immobile. They overcame this by tinkering incessantly with mechanisms to regain their mobility, and this ultimately is how they became an Artificer. By 3rd level, the prosthetics now have an independent mind.

In game terms, this functions exactly like a mount, except it's a "permanent" mount. I changed three thing to make this work: allow the Medium character to "ride" its Medium mount; if they dismount or the Defender falls to 0 HP, the character is considered prone; when "mounted", the character can move the Defender on the character's turn, rather than on the Defender's turn. In all other ways, it works exactly like a mount.

Overall, this is likely a decrease in versatility compared to a Small character riding a Steel Defender as an actual mount, but I think it's worth it for the flavor.

Sounds like the Armorer subclass with extra steps

yeah that would work

both of those ideas 

BTW how do you get the "Acolyte" title

Quote from PickleArtificer >>

BTW how do you get the "Acolyte" title

Post more. Titles are rewarded at specific post counts. There's a pinned thread about it somewhere I believe.

Back on topic though. I love the design of the Spidertron from Factorio. It's roughly what my devilish-looking Tiefling Artificer's SD looks like.

I'm using a homebred that allows upgrades for the steel defender so I'll have mine look like a kodiak bear and it'll rear up then sit back as I use the forge in its stomach to craft my weapons for my party and he can pull a cart by himself.  Super awesome uses to be had

I am going to try out using my steel defender as a wheelchair as see how that plays out.

I just wondered if the steel defender can transform his appearance, for example like a Transformer, animal form and humanoid form. 
I was also thinking if it could simply have hidden parts, like wings that pops out only in specific moments.

Not sure about the limitations here.

For the roleplaying part, is it possible to have both Homunculus and Steel defender? I'm gonna explain, I don't wanna use the homunculus to fight, only the SD, but I want to use the homunculus mostly for the roleplaying like a small creature to assist me like a tool and the SD as a proper companion.

There is nothing stopping you from taking the Homunculus Infusion as a Battlesmith - Yurei even posted about it and why it can be usefull even in combat.

As for transformations - mechanically I'd say you could implement that , but the SD is still always the same statblock, but that would be up to your DM. Popout parts could be a cool idea for flavoring the spells you cast on the defender - cast Featherfall - parachute - cast Dragons Breath (still think it should be on the artificer spelllist) - flamethrower, cast Fly -> wings.

Nothing's really stopping you from having your Steel Defender transform cosmetically, though as Sutlo points out there's no mechanical benefits (or drawbacks) for doing so.  Nevertheless, only a jerk DM would tell you your Defender couldn't have different forms for different tasks, especially as the Battlesmith's "Steel Defender" class feature explicitly allows you to change the thing's form when you make or remake it.  So long as you're using the freedom for Good (i.e. flavor, roleplaying, and fun), you should be good to go.

And since Sutlo brought it up...yes. You can absolutely have both the Homunculus and Steel Defender, and there's a lot of very good reasons to do so.

My battle smith, Palias Silverspringer, is starting out with a mechanical capybara dubbed Marva (an animal he encountered during an otherworldly experience or what he believes may have been passage through a Thelanis manifest zone, or perhaps an intervention by an aspect of the Forge Maiden who spared him from death during an explosion aboard an airship outside of Cyre during the mourning.)

Design inspiration taken from Garreth Jackson’s Artwork: https://garreth_icartworks.artstation.com/projects/wbNZV

Will be similar in design and shape, but with golden brass panels sporting nature motifs and filigree inspired by Palias’ own artistry, the steel gardens of his (now lost) home of Estos as well his otherworldly encounter. 

For my steel defender that’s a eight legged humanoid with cannons for arms.

My artificer had an older brother who protected him when they grew up. The brother joined the military when he came of age and died in a bandit raid. So to deal with his grief he poured his heart, soul and memories of his brother into his SD. SD was created in the image of older brother, now the SD fills the roll of protector to my artificer, just as the brother once did.

Quote from Smeems >>

My artificer had an older brother who protected him when they grew up. The brother joined the military when he came of age and died in a bandit raid. So to deal with his grief he poured his heart, soul and memories of his brother into his SD. SD was created in the image of older brother, now the SD fills the roll of protector to my artificer, just as the brother once did.

I've got to ask. Tadashi, is that you?

Love the narrative component of this!

My dragonborn Battle Smith has a Steel Defender she calls Chomper.  It resembles a six-foot long crocodile, but it's got longer legs that project directly under the body, a relatively short tail (compared to most crocs), and a head based off Kaprosuchus, making it deeper and broader than most crocodiles.  Inside the snout it's got a series of independently-moving blades that tear apart targets when Chomper bites.

I’m planning on playing a warforged with a defender that looks like a copy of himself. 

Quote from Genics >>

I’m planning on playing a warforged with a defender that looks like a copy of himself. 

 That's kind of hilarious