Do you think the Galra took Shiro's arm or Shiro lost it in battle and they replaced it?
oh I 100% think they just took it. For one thing, Shiro’s Escape outright proves it. We see Shiro on a surgical table surrounded by things these things that look like giant saws and when he’s crowded by the medics he’s clearly hyperventilating. He’s been here before. And he literally says, “You took my hand. What more do you want?”
Shiro also isn’t the only one with prosthetics or mechanical augmentations. There’s the guard in Beta Traz with his injections, Sendak’s arm and eye, even Rolo’s leg–ect. And Rolo also doesn’t say something like, oh, I lost my leg in a battle against the galra. He says, “I managed to escape, but not before I lost something,” which implies they took it while he was still a prisoner. Shiro agrees and adds, “I know exactly how that feels.” Again, it sounds like the galra were very deliberate about all this.
There’s also Sendak’s whole attitude about it. He views these mechanical components as “upgrades”–as if losing a limb and having a weaponized prosthetic forced on you is some kind of reward. Something you should be grateful for.
Sendak also says, “I see you’ve spent some time with the druids. They do love to experiment.” And knowing the druids, they’re not just gonna wait until someone conveniently loses an arm to test-run their new tech. There’s also this idea that galra prosthetics are better than flesh limbs. So as far as they’re concerned, they’re doing you a favor. You were already in need of medical attention even if you were “fine” because however you are now, they can still “fix you” so that you’re superior. “Too bad you didn’t get the latest model,” implies that Sendak’s arm is something to be sought after, an “upgrade.” This idea is further perpetuated when Sendak tells Shiro, “You’ve been broken and reformed. Just look at your hand. It’s the strongest part of you. Embrace it.”
Consider the little lizard that Haggar transforms into a monster. Like, that wasn’t a lizard that was suffering from some weird disease or anything. It was a perfectly healthy lizard and didn’t need any kind of treatment, and she still turned it into a monster. Basically, all the prisoners to her are like little lizards. They’re weak and useless as they are but if you work some magic and invest enough quintessence you can create a powerful weapon.
If you consider everyone the druids experimented on to be robeasts in some capacity–beings that were corrupted with refined quintessence and mechanical augmentation–then I think Shiro’s unfortunate circumstances make more sense. There aren’t all these high ranking galra and generals walking around missing eyes or limbs due to just bad luck in a fight. They were singled out as potential candidates to become these super soldier type robeasts.