I had something weird like that happen too: a shiny Magikarp came along through a wondertrade, and he looks reasonable enough with averaging stats.
Bu-uuu-ut... I saw the guy who wonder-traded with me in my festival plaza, and his recent activity points to nothing but magikarp wondertrades. So... Were they all shiny? Did they all come with Rare-candy? I just get so superstitious. I'm just not gonna play with him for now.
As for the Ho-oh, man this looks weird. Just be careful however you approach this - keeping or taking, this looks like a real sticky situation.