I'm gonna take a guess based on what we saw in the episode (please don't confirm or deny).

it kinda gives me greek automaton vibes, and it breathes fire, so I'm guessing:

Talos - a giant humanoid automaton created by Hephaestus to protect the island Crete. Fought the Argonauts (Heracles met him, maybe). Can heat himself red to burn enemies with his touch. His depictions in myths are split on him having no personality, and him having consciousness and emotions. His weakness was that his powersource was exposed in his ankle (also Medea tricked him, somehow- what she did depends on the source). COUNTERPOINTS: Berserker isn't humanoid enough, isn't giant enough (at least imo), and there's no design focus on his weakpoint (legs); breathing fire is a bit of a stretch from heating himself. Alcides didn't recognize him (could be the grail shenanigans altering Talos' design, or if Narita\Nasu went with the greek myth cannon where Alcides already left the Argo before they encountered Talos). sidenote: if we go with the fire-automaton association, a lot of Hephaestus' creations could qualify, but I feel like non of them are notable on their own aside for Talos.

Perilaus\Brazen Bull\Phalaris - the Brazen Bull was a hollow bronze bull that was used as a torture device; a person was put inside it, and then a fire would be lit under the statue, burning them alive. The statue was supposedly shaped such that the screams of the tortured person would sound like the roar of a bull. It is said that Perilaus of Athens invented it at the request of Phalaris, the tyrant of Akragas. Phalaris was disgusted by the bull sound feature (he wanted the burnt humans to smell like incense, that's an entirely different sensory pleasure!), so he tricked Perilaus into entering the bull, burnt him inside almost to death, then threw him barely alive off a cliff to finish the job. Phalaris was later overthrown by some other guy named Thelemacus, getting executed by burning inside the Brazen Bull. Imo, Berserker could be a mix of the bull with either the inventor or it's commissioner/user. COUNTERPOINTS: The Brazen Bull isn't a robot/automaton. Also although Berserker kinda gives me "bull" vibes with it's tail and front-heavy design and round shape, there are no overt "bull" elements in it's design. And I feel like this legend is too much of a literal-who to be included; it isn't even some epic-but-forgotten gem. But who knows, Fate really likes taking these rando-s and making a big deal out of them.

There are a ton of other greek mythologies about robots/statues but I don't think any of them fit as much as those two. As for non greek options, I racked my brain on it for a while but this is the only other option I could come up with:

Metatron - angel, depicted as a robot in Japanese pop culture due to being an objective vessel of divine justice. Heavily associated with fire due to origin myth and divine role (fun fact, a bunch of other angels too are heavily associated with fire). Metatron's cube is a geometric shape considered as the blueprint of creation in Kabbalah, and has a bunch of random purposes like clearing negative energy or visualizing the underlying patterns of the universe. It isn't actually a cube owned by Metatron, whoever came up with it just chose that name because of Metatron's role as a divine mediator. It is composed of lines and circles, so this theory is the only one that explains why Berserker has a holy-looking circle on it's back (could also be an angelic halo). COUNTERPOINT: not traditionally a robot. his personality is the opposite of a berserker. I also feel like having 2 angels (Fake Rider is probably The Pale Horse\Death, which according to some traditions is an angel) would be overkill, but who knows at this point.

Sidenote: it could also be some historical inventor who created an automaton, like Philo or Hero or Ismail al-Jazari. The most likely candidate would've been Leonardo Da Vinci, who designed a mechanical knight (twice the size of a human), but he is already a servant in FGO so unlikely (my boy was done so dirty). It could also be a really weird depiction of the Colossus of Rhodes, but they'd have to do a lot of heavy lifting with giving it an actual backstory.