Battle — Siege // Creature — Rhino
Battle — Siege
(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, create a Treasure token. Then you may discard a card. If you do, draw a card.
Creature — Rhino
Trample
When this creature enters, you may discard a card. If you do, search your library for a land or battle card, reveal it, put it into your hand, then shuffle.
This card uses a replacement effect ('as [it] enters' or 'enters with') to modify how the permanent enters. This is NOT a triggered ability — it can't be responded to, and effects like Panharmonicon or Torpor Orb don't affect it.
This card's ability triggers when it enters the battlefield, not when it's cast. Effects that double ETB triggers (like Panharmonicon) work here. Countering the spell prevents the creature from entering, so the ETB won't fire.