Big mobs without behavior/texture pack

So I teleported to a woodland mansion, burning all the trees near it(don't ask me why lol) and then all of a sudden this chicken came up to me.. it was much larger than a normal chicken. I did not have texture/behavior packs on. Near the chicken, there was a normal chicken, so that means it is probably a fluke. But still, there is this huge chicken! I don't know how to reproduce this thing as it just came out of nowhere.. maybe teleport to a woodland mansion and burn the trees?