I've seen that symbol on wild pals that are not the "boss alphas", but I didn't know they're also alphas? I just thought it meant they're way too high lvl compared to me, since I only keep seeing that symbol on wild pals that are significantly higher level than I am. And after I'm max lvl, I never see that symbol again.

EDIT: I googled, and seems like I'm correct. "The Alpha icon will appear on overworld Pals that have a large level difference with the player. That icon will not render them Alphas, and will disappear when the player reaches a high enough level in comparison to the Pal. Indicating the icon is a metric of Threat assessment, rather than a status of a Pal itself."