Transpeople have always existed we just did not see them as other humans historically had a habit of murdering them. Naturally this kept them out if sight and the modern transrights movement is attempting to change this.

Your second example is not a real thing and if it were and adult identifying as a child would be delusional because they are not a child. Childhood is not a social construct like gender is so the situation is different.

Rachel Dolezal was the person you were thinking of. While race is also a social construct she understood her ancestry to be different and pretended otherwise to the point if asking her brother to not point out she was white. That means she identifies as a white person on some level whereas transpeople do not identify as their assigned birth gender.

Three different situations