The whole story was someone (I do think owner's grandkids since they were really big troublemakers in barn) spit something on both of my horses (something very stinky). My mom saw the stinky manes in the afternoon, but when I came in the evening (2 or 3 hours later) 1/3 of the mane (in mid of neck) was gone on one of them (and it was obviously cut, not chewed or anything). Second one still had that stinky stuff (like chewed hard candy or something) all over her mane, but noone in world except me could catch her that time (and especially cut), so I assume that's why she stayed untouched (took me 2 hours to clean her mane though).
I'll try to pull to see what will happen. I'm afraid it's just still way too short there, so may be cut is the only possibility. :(