Boros Aggro is a fast, creature-dense deck with Momo, Voice of Victory, and dragons that pressure quickly, so we want cheaper, more resilient threats rather than our clunky 9-mana Sunderflock, which also has the downside of bouncing our own board if we're racing. Colorstorm Stallion comes in as a hasty 3/3 that scales with our spell-slinging plan and can close games faster than we're used to. Slagstorm's 3 damage sweep mode is strong against their 1-3 toughness creatures like Momo and Sarkhan, Dragon Ascendant, helping us stabilize. Spell Snare drops since few of their key cards sit at exactly mana value 2, trading it for more proactive damage-based answers.