Seeing as this is post incident this advice has little value but I will render it all the same.
The first and most important question always is, is "Do they have a need to know?" Will telling them improve the relationship? Is it worth risking rejection and hurt? Is it something that one can go on without doing lest it weigh on their conscious?
If the answer to any of this line of questioning is "No.", the individual should reflect on what is driving that urge and what consequences would come from it; the weighing of pros and cons. I mean that sincerely too, that this is critical because what one might mean very honestly and for the sake of being noble as well as forthright does not mean mundane persons will receive this so well.
The second of these things is to frame it in a way that people are familiar to and can understand in modern parlance. The idea of therians and therianism, the psychological and spiritual halves respectively which also apply in more broad concept to otherkin as a whole, is no revelation. The ideas of being more than human or desiring to be and aspiring to be, to become "divine" in a way is prehistoric. Which is to say this is nothing they cannot understand, rather it is an exercise in presenting the experience.
The best ways in which I have come to explain it to people is that is akin to reincarnation, the idea of the transmigration of soul into a new form. That another avenue is that one has a spirit animal and that it is them, that they are one in the same, which is a very common belief in many indigenous peoples' beliefs. Another is that it is a "power animal" or divine presence, something which sparks them and is with them, part of them - fundamentally the Holy Ghost in its original, unaltered understanding in Christianity is fascinating in its way of being similar in that it is an ethereal being and thought who animates the person who welcomes it into their heart. Obviously one can see the supposed disparity is not so much as thought and that there are many explanations as well as interpretations, a few of which I mentioned here briefly, at least in the spiritual sense whereas in the psychological one the answers tend to be a bit more dire.
The therian and otherkin phenomenon all have telltale signs of identity disorders or really more specifically tropes of dysphoria and or dysmorphia. While not a recognized disorder, by which I mean the major medical condition listings for and in billing to insurance, "species dysphoria" is a subset of the greater whole and is not widely known because of its relatively rare quality - extreme infrequency - and that many of the hypothetical sufferers are functional compared to other dysphoric persons. I should also note dysphoria is a pervasive, underlying, relentless, consistent sense of unease and discomfort, in this case relating to the reality that the individual perceives themselves to be something other than human and are troubled by this subconscious nagging that some parts, alternatively all of them, are not right. Only a licensed clinical practitioner in psychology or psychiatry ad therapists should make the ultimate decision about this but it is a very real thing, something tangentially related to gender dysphoria - formerly "Gender Identity Disorder" or "GID". I strongly advise that if this is the case or if one believes it to be that they seek professional assistance and in this case, with the help of their parents. I say this because major anxiety, depressive, and mood disorders - as well as other conditions that may be inhibiting the best quality of life - are likely to manifest or worsen.
Allow me to state the psychology of "otherness" should not be treated simply or completely as malicious to the individual. Some of us, myself included, draw from twin wells - that psychological and that spiritual. It is a driving force to not only live up to what I could and should be in the world, even if I only am in the mental one, but also that I am called by divinity to be more than I am. It is an aspiration and path to take for life improvement and self-discovery. It is part of the process of actualization and individuation, the process of learning and being who one is from all their disparate parts into one sum within their mind and spirit, good and bad.
i hope that this post provides you with something, that it helps explain and open those doors needed, helps lead to those answers required,
@Leaniz.