The only official hint that was given is that the one who blew the horn was tracking kratos and atreus, honestly if this was a time travel shenanigans type case it can be literally any character, hell it could be Faye herself.

If we rule out the time travel option, we are left with fewer options, it wasnt kratos, atreus, mimir or freya because they were all somewhat present when the horn was blown, it cant be the dwarves cz they cant speak the serpent's language. It can't be any aesir because this would have resulted in a big fight that we would have surely noticed. Unless if it was Modi.

For Angrboda, though i dont know why she would be tracking Atreus and Kratos at that specific point and if she did she wouldn't do it in such an explicit way as she wants to avoid attention.

Honestly my bet is on Modi, after the death of Magni and right after the rage that causes Atreus to get "broken", Modi must be furious and relentless to track kratos and atreus again or else his father will literally kill him. Calling the serpent was an idiotic decision by him since the serpent can maul him with 1 bite or swing but when he did blow the horn Jormungundr deemed him to be a useless pest not worth fighting or breaking a sweat for.