In the attached image, I was playing MTG Arena Alchemy. I haven't played MTG in some time, but I cannot figure out how one of the opponent's creatures gained Trample.
He had just laid a land (it was the only card in his hand after the draw), then he attacked with a Spirit of the Aldergard with stats of 12/5. He had a Master Symmetrist in play, which gives Trample to creatures with equal power and toughness. I blocked with a 5/5 creature and the opponent's creature killed mine and did seven damage to me. After this occurred, I careful scanned each card to see what I had missed, but nothing should have given the creature trample.
There must've been a bit more that happened between your screenshot and the end of the game, since you said you blocked with a 5/5, which isn't present in the picture, and that their Spirit of the Aldergard was a 12/5, when it's only a 11/4 here. But going from 11/4 to 12/5 is +1/+1, so that's not just +1/+0 from another snow permanent, and if it got a +1/+1 counter from somewhere, the Gnarlid Colony would give it trample.
Sorry, I wasn't clear...my opponent had an additional Spirit of the Aldergard that was 12/5. After it dealt the Trample damage, it died. So the Gnarlid Colony was what gave it Trample. I didn't see that, thank you.