TL;DR We don't know, but really who can tell if anything outside of Þórr, Óðínn, Frigga, Baldr, Freyr, Ullr, and Týr exist pre-Christianization.

Skaldic verse, the Merseberg charms and place-naming conventions are the best ways for identifying extant pre-Christian gods, and Loki doesn't show up as Loki in any of them.

Now that said, sure, maybe he was around. We have attestations of Lóðurr in Reginsmál, and Ursula Dronke conflates him with Loki since Lóðurr appears with Óðínn and Hœnir in Haustlöng, and Loki is given the epithet "Hœnirs friend" in several sources. Jan de Vries and Georges Dúmezil also have conflated the two.

We also have 10th century, either just pre- or just post-conversion archaeological finds, like the Snaptun Stone and the Kirkby Steven Cross and Gosforth Cross which depict figures who might be Loki.

On the Snaptun stone, it has a man with what appear to be stitchmarks on his mouth, which conforms with the story in Skáparmál where the Ivaldsonnar stitch him up because of the whole "turning into a fly to bother them while smithing for the Æsír" thing. On the crosses, there's a bound figure under serpents who might be Loki or might just be the antichrist, as the symbolism there is similar.

John Lindow is in the "he's post-conversion or at least not Norse" camp, as he points out similar motifs in Loki and his children being bound to those found in pagan religions from the Caucasus, as well as the bound antichrist symbolism.

There's lots of weird stuff with Loki, lots of people have written a lot more about him than just about any other Norse god, so take a look at the following for more info:

• Arvidsson, Stefan (2006). Aryan Idols: Indo-European Mythology as Ideology and Science. University of Chicago Press.
• Haukur Þorgeirsson (2011). "Lokrur, Lóðurr and late evidence". RMN Newsletter. 2: 37–40.
• Lindow, John (2001). Norse Mythology: A Guide to the Gods, Heroes, Rituals, and Beliefs. Oxford University Press.
• Madsen, Hans Jørgen (1990). "The god Loki from Snaptun". Oldtidens Ansigt: Faces of the Past. Det kongelige Nordiske Oldskriftselskab.
• von Schnurbein, Stefanie (2000). "The function of Loki in Snorri Sturluson's 'Edda'". History of Religions. University of Chicago Press. 40 (2): 109–124.
• Simek, Rudolf (2007). Dictionary of Northern Mythology. translated by Angela Hall. D.S. Brewer.
• Turville-Petre, E. O. G. (1964). Myth and Religion of the North: The Religion of Ancient Scandinavia. Holt, Rinehart and Winston.

Loki has been a hot topic over the years in ancient Germanic studies. One of the very few things that can be said for certain about Loki is that he wasn't a late invention: He's attested in some of the earliest Old Norse sources, such as Ynglingatal (generally considered to have been authored by the skald Þjóðólfr of Hvinir, and this source also happens to be the earliest clear reference to Hel, referred to her as Loki's daughter). Loki can be found referenced here in there up until fairly recently in Scandinavian folklore, including outside of Iceland and Norway, and he's probably depicted in stone on a variety of objects. His wife, the goddess Sigyn, is also generally considered to be quite old.

Modern scholarship generally considers Loki's place in the North Germanic body of myths to be similar to that of other entities or deities who fulfill a similar role elsewhere, such as Prometheus in Ancient Greek myth. For example, both Prometheus and Loki invent things useful to mankind, run into problems with the gods, and are bound and suffer in similar manners. Similarly, Prometheus is not known to have been venerated among the ancient Greeks like, say, the Olympians, and there remains no known cult surrounding Loki. It happens that Loki is in remarkable ways quite similar to Odin, too, which has yielded a lot of discussion.

Loki isn't attested in other Germanic branches, and so—unlike, for example, Hel—scholars can only secure him within the North Germanic branch and he may have been an innovation specific to North Germanic mythology. However, the other branches provide very little in terms of comparable material and when we do get a very glance among the mythology of other Germanic branches, like in the Old High German Merseburg Charms, the Old English Nigon Wyrta Galdor, and the Lombardic account of Odin and Frigg, what we find is frequently incredibly similar to the North Germanic material. In short, the classic absence of evidence is not evidence of absence applies. There may have been a Proto-Germanic Loki figure at one point, we just don't know, but some scholars have proposed that both Prometheus and Loki represent an earlier figure (or motif cluster) inherited from a common mythological ancestor, perhaps as far back as Proto-Indo-European myth.

For more discussion on Loki and related sources, check out these sources:

• Lindow, John. 2002. Handbook of Norse Mythology. Oxford University Press.
• Orchard, Andy. 1997. Dictionary of Norse Myth and Legend. Cassell.
• Simek, Rudolf. 1996. Dictionary of Northern Mythology. D. S. Brewer.

Note, however, that some are more up to date than others and Simek, for example, has a habit of inserting his opinion as fact (and not updating his entries, with many dating to around the 1970s).

And here's a good paper discussing various theories surrounding Loki:

• Von Schnurbein, Stefanie. 2000. "The function of Loki in Snorri Sturluson's 'Edda'". History of Religions. University of Chicago Press. 40 (2): 109–124.

However, all of this predates Haukur's paper on Loki, which is the most important piece of scholarship on the figure in a long time. It can be read online:

• Haukur Þorgeirsson. 2011. "Lokrur, Lóðurr and late evidence" (PDF). RMN Newsletter. 2: 37–40.

Good luck!

The main 2 sources for Norse mythology, the Poetic Edda and the Prose Edda were both written/complied in the 13th century, in post-Christian Iceland. The Prose Edda was written by Snorri Sturluson during the early 13th century as more or less of a guide on how to write old Norse poetry, while giving examples as the myths he knew. The Codex Regius however is a manuscript containing the 29 poems commonly designated as the Poetic Edda, also compiled sometime during the 13th century. These poems are often quoted in the Prose Edda, and otherwise can be dated using linguistic evidence to earlier times, the oldest being Atlakviða, possibly dating to as early as the 9th century.

So technically almost none of what we know of Norse mythology is from before the Christian missionaries, as the 2 main sources are from 200 years after the Christianization of Iceland. That doesn’t mean that the sources are invalid or tainted by any means, and there are a whole manner of different opinions on how much they are affected. In a sense, Snorri is a secondary source, trying to interpret the same poems as us, and the Poetic Edda/Codex Regius is a primary source, given how it can be traced back linguistically to before it was compiled.

To answer your question, the poem Vǫluspá, one of the most important sources for Norse mythology, is probably from the 900s by linguistic evidence. In stanza 35 of Vǫluspá, a völva (seeress) tells Odin that she sees Sigyn sitting unhappily with her bound husband Loki under a grove of hot springs, and in stanza 51, during the events of Ragnarok, Loki appears free from his bonds and is referred to as the "brother of Byleistr”. Another poem considered old because of the word use and alliteration is Lokasenna, which centers around Loki insulting the other gods. Those are probably the best evidence we have that Loki is “canon” in Norse mythology