We've watched this show for long enough to know that a big reveal like that always comes with more questions. In this case, we're sure not sure who Ali's accidental almost baby-daddy is.
What we do know is that Ali met him at Cape May, while she was hanging with CeCe (Vanessa Ray). Whoever he is, she seemed deeply scared of him — she told CeCe he'd "kill" her if he found out, and she sounded like she meant it literally.
Who could this frightening, unknown beach hottie be? We have a few theories.
Detective Wilden (Bryce Johnson): The show seems to be pointing in this direction, with Emily finding a photo of Wilden at Cap May, too. Wilden's a creep, so we wouldn't put it past him, and this would certainly explain his obsession with the Liars. If he did have something to do with Ali's death because he found out she was pregnant, it makes sense that he'd want to pin it on the Liars. Alternatively, if he had nothing to do with Ali's death but had been hooking up with her, that might be why he's so invested in the case.
Ian Thomas (Ryan Merriman): We know Ali and Ian were romantically linked; perhaps she used the "beach hottie" nickname to hide his identity from CeCe. There's still a lot we don't know about Ian's involvement in Ali's death, and this would also be a good way to explain why Melissa (Torrey DeVitto) seemed to hate Ali so much. On the other hand, if Ali was so scared of him, why was she still seeing him, and apparently happy about it?
Someone we don't know: There's always room for new sketchy characters in this story!
No one: Ali seemed genuinely upset in that flashback, but it's possible she was lying about her pregnancy as part of a larger game. Or perhaps CeCe was lying to Emily for her own reasons.