It pains me to say this, anonymous, but from personal experience, it is very, very difficult to change people’s minds once one gains a certain reputation. If someone is adamant about their perceptions of others, it usually takes a significant amount of effort to make them think otherwise. This goes especially with pokémon. Most people are terrified of gyarados, even though trained gyarados are docile—sometimes even friendlier and more loyal than trained arcanine. Likewise, haxorus is a species known for its gentleness, yet people are terrified of it because it’s a fierce-looking dragon, even though Unova alone hosts at least one far more violent species.
The best that you can do is show others that your haxorus means no harm. Let her spend time with you outside her poké ball in public areas, such as that park. Keep her calm when others approach, and if someone acts aggressively towards her (either aggressively affectionate or simply aggressive), help her to remain stable through the encounter. Yes, you may be tempted to tell the human off for being too aggressive, but this may escalate the situation. If possible, try to explain why she needs space—because she is, after all, a pokémon—but be as polite and clear as possible when you do so.
When pokémon approach, have them make the first move. Train your haxorus to react more than initiate, but train her to assume relaxed stances when she wishes to interact with others. (For a haxorus, this means relaxed limbs, head bowed, and jaw loose but not open. The less tense your haxorus looks, the less she appears to be preparing for an attack.) Oversee the interaction, but allow your haxorus to show how gentle she actually is. If trainers see their pokémon interacting with yours without incident, they’ll be a little more open to her presence.
But most importantly, as her trainer, it’s important for you to initiate conversations with others. Have your haxorus present and by your side, and if your haxorus seems interested in another pokémon, approach the trainer yourself and explain to them that your haxorus is gentle and would like to interact with their team. Good trainers will either understand that a fierce-looking pokémon isn’t necessarily a violent one, or they’ll be more receptive to learning about this distinction. Trainers who aren’t entirely worth your time will be less likely to listen to you.
Either way, always reassure your haxorus that if you do encounter a not-so-open-minded trainer, it’s the trainer who’s in the wrong, not her. With some effort on her part to show her gentle side and some effort on yours to help her build connections with your community, there will eventually be pokémon who will accept her for who she is, and the opinions of those who refuse to see her for her gentle nature won’t be important. But the point is, never let those negative opinions hinder you from reaching out to the people who are more willing to accept you.