I’ve always loved the weird corners of the Nasuverse where a single tweak makes everything feel new, and ‘proto-saber’ is exactly one of those tasty what-ifs. In the most straightforward sense, when people say ‘proto-saber’ they usually mean a prototype or alternative incarnation of the Saber-class Heroic Spirit — the kind of Saber who shows up in early drafts or spin-offs rather than the mainline 'Fate/stay night' Artoria we all know. The clearest example of that is the whole 'Fate/Prototype' branch, which was Kinoko Nasu’s earlier draft of the Holy Grail War and intentionally reimagines key roles and relationships.
That matters because the Nasuverse treats all of these as parallel possibilities rather than a single tidy continuity. So ‘proto-saber’ isn’t one universally canonical person across every Fate story; she (or he, or different design) is canonical inside their own alternate work. I love hunting down those differences — different personality beats, different Noble Phantasms, subtle legend reworkings — it’s like seeing a familiar painting from another angle. If you’re curious, check out 'Fate/Prototype' materials and related short works; they’re short, strange, and give a cool perspective on why Artoria ended up the enduring Saber figure.