Stampeding Horncrest

• WWizards of the Coast2018-01-19
  Official Ruling
  If it’s the turn Stampeding Horncrest comes under your control, and it loses haste after being declared as an attacker, it will continue to attack. It won’t be removed from combat. On the other hand, if it loses haste before your declare attackers step, it won’t be able to attack.

Rulings via Scryfall