Stampeding Horncrest — ruling 1

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.

Stampeding Horncrest : règles, fonctionnement et interactions (MTG)

Stampeding Horncrest is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This creature has haste as long as you control another Dinosaur.

How it works : Stampeding Horncrest

Original English text — Wizards of the Coast / Scryfall

#1January 19, 2018· Wizards of the Coast
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.
No other card is named in this ruling.