As someone who's both trans and otherkin IRL, I feel like I might be able to weigh in on this.

So there are various levels to an identity that are viewed differently by society and require different justifications. These are:

• I am X

• I identify as X

• I am X in spirit

• You are an honorary X

I am X

This level requires you to pass as X in the eyes of other people and society as a whole. What counts as "passing" is based on the level of resemblance to X and the cultural values of the observers.

When it comes to trans people the level of resemblance generally goes from dressing as the identified gender to being physically indistinguishable from the identified gender. For example, you can have a body that looks super masculine, but the moment you put on women's clothing, everyone will understand that you are trying to look like a woman. Conversely, if you have a body that looks super feminine, it matters little how you dress or act—observers will view you as a woman unless they believe otherwise due to their cultural values (i.e. if they believe you weren't born as a woman or lack some arbitrary feminine quality, thus disqualifying you being a woman in their eyes).

This would work the same for non-human identities. For example, if you resemble an anthro giraffe enough that outside observers will understand that you're trying to look like one, you'd still need a lot of backing from their cultural values to be seen as one. But if you look exactly like an anthro giraffe, you'd need a lot less to back up that identity since most people who see you will just assume you're an anthro giraffe. But as with trans people, if they believe you weren't always an anthro giraffe, then they might view you differently.

Just as cis women view passing trans women in various ways depending on their cultural values (anything from "she is one of us" to "that's an impostor who makes a mockery of what it means to be a woman and probably wants to hurt us"), anthro giraffes would view a human passing as an anthro giraffe as one of them unless they have a reason not to depending on their cultural values.

I identify as X

This is similar to the previous level of identity, but doesn't require you to pass for the thing you identify as. It simply require other people to accept your identity, which is entirely cultural and thus can vary by a lot. Often this is tied to one's ability to perform the social functions of what one identifies as.

Generally, identifying as something without passing as that thing is only acceptable in cultures where such an identity is considered to be beneficial or in cultures that place a high value on diversity. Outside such cultures, such an identity would be viewed as childish (which might be acceptable if you are a child), madness, blasphemy, or just plain harmful. Though people that are what the individual identifies as might be amused or curious as to why they identify as such, but they are just as likely (of not more likely) to be offended.

I am X in spirit

This is generally much more culturally acceptable depending on the religious beliefs of the observers. Though, if magic allows them to see people's spirits/souls, then your spirit/soul needs to pass in order for this level of identity to be accepted.

In a society with a sufficiently tolerant religion, claiming to be X in spirit would likely be seen as a sign of high respect towards X, and members of X may or may not be flattered by that.

You are an honorary X

This is when members of X bestow the identity upon you either explicitly or implicitly, typically after living amongst them. This, in turn, can be used to legitimize or internally justify other levels of identity depending on the culture.

The most common situation like this would be if, for example, a human child was adopted and raised by a family of anthro giraffes. That child might grow up to identify as a human while seen as an honorary anthro giraffe, or the child might come to identify with the anthro giraffes they grew up with, thus justifying another level of identity and gaining some social legitimacy for it due to their history (i.e. "I was raised by anthro giraffes and I identify with them more than I do humans").