- Schuyler Bailar — NCAA Division I men's swimming. Assigned female at birth, transitioned to male and swam for Harvard's men's team from 2015–2019. He was the first openly transgender athlete to compete on an NCAA Division I men's team.
- Iszac Henig — NCAA Division I swimming. Henig was an All-American swimmer on Yale's women's team before transitioning and beginning testosterone. He then switched to Yale's men's team for his senior season and competed directly against male swimmers, despite knowing he would rank much lower in the men's division. That's a pretty strange way to go about transitioning if the motivation is supposedly to gain an unfair athletic advantage.
- Mack Beggs — wrestling. Beggs is a transgender boy who wanted to wrestle against boys in Texas. Texas rules required athletes to compete according to the sex on their birth certificate, so the state forced him to wrestle in the girls' division. He won girls' state championships in 2017 and 2018 while taking prescribed testosterone. His case is particularly relevant because people sometimes cite him as an example of a transgender athlete dominating girls' sports while leaving out the fact that he wanted to wrestle boys.
- Chris Mosier — men's triathlon/duathlon. Mosier was assigned female at birth, transitioned to male and competed internationally in men's competition. In 2015 he became the first openly transgender man to make a U.S. men's national team.
- Patricio Manuel — professional men's boxing. Manuel transitioned from female to male and became the first openly transgender man to compete professionally in men's boxing in the United States. He's fought cisgender male professional boxers.
The Mack Beggs case is probably the single best answer to the your question.
Beggs essentially said: I'm a boy and I want to wrestle boys.
Texas said: You were born female, so you have to wrestle girls.
He then won two girls' state championships.