Let's say I have a Lone Rider and I've gained enough life to trigger it. Opponent is holding a Merfolk Trickster and wants to stop Lone Rider from transforming. Can opponent use Merfolk Trickster on my end step (after Lone Rider has triggered) or must opponent cast it in the main phase before the end step?
Usually removing the source of the trigger doesn't stop the trigger, but in this case, after Merfolk Trickster revolves, Lone Rider has no abilities anymore. Does an ability-less Lone Rider still "know" it can transform?