Memories are not necessary to be otherkin. I have a few kintypes that I don't have memories of, just strong consistent feelings that they are me.
Being otherkin is first an identification as non-human. The specifics of that non-humanity falls to each individual in terms of what one determines that to mean. Everything else kind of comes second to that in one's own exploration of that identity--memories included.
For some, especially some spiritually based kinfolk, having memories validates the kintype, but it's not the only factor in determining what is a kintype or not. The most important thing is identification as the kintype, but how one ends up determining that is usually varies based on their experiences and how they approach exploring their identity.