It's not suitable. Fanfiction should be a form of creative expression that respects the original work and its fans. Gay sex fanfic in the context of 'hunterxhunter' may be offensive to some viewers or readers. It's better to focus on more positive and inclusive themes that are in line with the spirit of the anime, like friendship, adventure and character growth.
Friendship is a wonderful theme. You could write about the deepening of the bonds between the main characters. Like how Gon and Killua's friendship evolves as they face more challenges together in the Hunter world. It could involve them having heart - to - heart conversations, supporting each other in battles and learning more about each other's pasts.
Mang is definitely not a horse. There's no common reference or understanding that Mang represents a horse. Maybe you heard it wrongly or it's a made-up term in a very specific context that's not widely known.
I've never come across the term 'mang bts'. It could potentially be a nickname or a code word within a particular group or community, but without more context, it's hard to say for sure.
It depends on what you mean by healthy. If you're talking about the ingredients and cooking methods, some of their offerings might be considered healthier than others.
I'm not sure exactly what Mang BT21 is. It could be a fan-made manga based on BT21, or a special edition or spin-off of an existing BT21-related comic.