Everyone's quick to judge just what a monster was, from how 'menacing' Armstrong looked from his bulging muscles that billowed down his torso to the massive green tail that extended from his waist. He was one of the largest mermen that Ed knew, and despite his friendliness, others still found him scary, including the sailors that hoped to spot a pretty mermaid and not a vastly powerful merman. Armstrong either didn't notice or chose to make up for it somehow.
Ed wasn't immune to being seen as a monster either as transformable Automail was still in its infancy, and few people knew how to handle a mer with a partially metal tail. Ed had had to go back almost constantly as a child until Pinako and Winry figured out how to make it right. He couldn't swim without it, as he missed half a tail, and when he did manage to, could only move in circles.
Edward was immensely grateful as it kept him from going in for surgeries everytime he either wanted to or needed to be in human form. Other mers usually just moved to land, went for regular surgeries, or simply needed help to move. Ed considered himself far too young to need that much help as it made him feel utterly useless. Winry and Pinako were lifesavers to Ed, and there was simply no way to go around that.
Truly, if one was a monster, you couldn't tell from thick bands of muscle or metallic body parts. Only, the heart can hold that much notice and attention to the fact, and the heart can never be fully read by any one person, though sometimes the small waves and trickles from it are revealed in the actions, trickles that might hint on the inner nature of that person.
Honestly, Ed may appear like a monster by how much light weight, waterproof metal was needed for him to move with ease through the water and across the land, but he'd never been the 'kind' of monster to fear. To be partially metal never could determine the true weight of a person's heart.
Alexander Louis Armstrong may appear to be a monster from how strong and powerful he was, that could strike fear into a person unaware that he did not mean anyone harm unless they really deserved it, and they really had to deserve it for a major fault to be beaten up by him. His kindness may not appear in the muscles at first glance or in the tail that could easily capsize a boat or two, but it's there, resting within his heart and exuding from him on all sides.
Honestly, a monster is not revealed from need of certain medical assistance or from how often a person noticeably works out. Truly, look beyond the muscles and see the shining hints of the warm heart underneath and glance past metal that keeps a tail moving through the water together and keeps legs and arms doing what they need to do, and see the beautiful hints of the heart underneath all of the struggles, the trials, and the difficulties. You won't regret the beauty that you can find there, even without yet knowing the full picture. It's there in every beat of the heart and every thrill of an excited smile.