People who have not encountered deities will not really care for this discussion. Those who have, and those who intend to try, are the ones who wonder. Assume a being comes to you, presenting herself as Isis. Her reality is not questioned -- after all, you *sense* her. Even if she is a figment of your imagination, she is real. *What* is she, though? Should you think of her literally, as the daughter of Nut, who gave birth to Horus in the papyrus swamps, or a being who just clothes itself in the image of mythical Isis to approach you? The answer is really of no importance at all -- the magical community is aware of that. What matters in these encounters is what you learn from them: the lesson is valid whether it was handed to you by God or by yourself. The speculation is interesting for its own sake, and I present it in this spirit. Let us then return to the question: What could your Isis be?
The first possibility is that she's just that: Isis, an ancient, divine being as the mythologies tell us. We can take the legends literally and accept that she had to collect the pieces of her murdered husband, or we can consider that the myth is a symbolical rendering of her story,that doesn't affect her goddess nature.
One step lower, it could be that Isis was an ancient figure (not deity) of importance, who later was deified by the people, and actually returned to earth as a spirit to fill the "job title" of Goddess Isis now created. Perhaps she even started out human and then for some reason grew to become a goddess, and only then did she present herself to human awareness and the myths arose.
She may have originated in an archetype instead of a human being. Drawing on the archetype of motherhood, the Egyptian population could have created the "paper character" of Isis. Then, seeing that the population needed this figure, an angelic being could have filled this shell to answer their needs through the image that worked for them. In this case your Isis would be an anonymous Angel who has been playing the part of the well-known goddess ever since she was needed on earth. This would mean anyone else who meets her, is facing the same being you are. However, this shell created by a population may be inhabited not by a single, constant being, but occasionally, according to individual needs.
In other words, it could be that one of your guides has borrowed the image of Isis to speak to you, feeling that this was how you, their pupil, needed them to appear.
Now what if the population had created, not a shell, but an egregore? This spirit, fed by belief and worship, could have grown to truly godly status and you'd be speaking with exactly what the legends say about Isis (since the legends would have created her), except she'd be younger, not older than them -- not pre-existent to humankind. In this case the egeregore is created unvoluntarily, and probably even unconsciously. It is possible however to create a spirit deliberately -- maybe your Isis started out as one person's thoughtform (inspired maybe by the old myths), and after being carefully fed and "raised", outgrew her creator's control or became independent at his death, and is now for all intents and purposes the Isis of mythology. This would again be a case where everyone who meets her, is meeting the same being. If you intensely wished to meet Isis however, it could be that *you* created this thoughtform -- or a shell that another being (Angel or guide) took advantage of to reach you.
Finally, she can be an aspect of the unconscious: either yours, whereas you unconsciously projected a part of yourself in her image in order to receive information from an apparently-exterior source (most of us trust more what we receive from others than what we come up with on our own), or and aspect of the collective unconscious, which can be tapped by anyone at anytime.
There may be many more possibilities I forgot, and I didn't even mention the one that God in person is taking on a face to talk to you without making you fall flat on your behind... There's also the option that the ancient goddess is and always was an Angel, transformed by poor vocabulary. All the theories I evoked are not mutually exclusive. They can all (or none) be true. The existence of the ancient goddess Isis does not exclude the existence of an thoughtform-Isis (if the idea the people have of her do not correspond to her real nature) or that of a "private" Isis that is a part of yourself. You may be discussing with Isis the Goddess, while I'm having a chat with an Isis created by an Egyptophiliac magician two centuries ago. It is probably impossible to figure it out -- but once again, what's the point? Your gut will tell you if you're dealing with a loving motherly being or an impersonal messenger. Further than that is just food for thought.