I think this is My Secretly Hot Husband.
Another lifetime, another hard life—Letitia’s misery continues as her uncle sends her off to the “Monster Lord” to fulfill an old pledge. Imp attacks! The three witches! A hideous masked devil hunter husband! Terrifying is an understatement. But as she becomes the lady of Halstead, the rumors fall apart. Lord Erden is caring, he likes bunnies, and that gasp perfect face under the mask! How can he not know he’s HOT? Can she help protect the castle from devils and help Erden see his true self?
Leticia Grey Halstead is an orphan, living in a fantasy world with horse-driven carts, noblemen and women, and monsters. She has memories of a past life in modern Korea, but keeps them a secret from most, as people with such memories are apparently tried as witches and burned at the stake. She has red hair, which is apparently associated with witches as well.
At the start of the story, she's married to Erden Halstead, a "monster lord" and demon hunter, who wears a horned black mask and is cursed to believe that he's "hideous and disgusting," despite possessing the good looks of a fashion model. Erden cares deeply for Leticia, and during a carriage ride in the opening chapter, tells her that he thinks they should get divorced since she's too good for him; she's definitely not in favour of that, however.
For the next few chapters, the story shifts to several years earlier, revealing the circumstances which led the pair to get married, involving a debt owed by the Greys to the Halsteads.
We also learn that Erden's castle comes under repeated attacks from monsters, and that the castle has a magical artifact, called Tirua's Tear, which turns it invisible and makes most people and monsters forget that it exists while he's away, and therefore unable to defend it personally.
Leticia does her bit to defend the castle too, helping to fight off attacks from imps and giant ants by using cans of butane gas as makeshift bombs, which leads others to think she might be a wizard. She also tricks giant stone golems who were planning to attack the castle into fighting and killing one another instead.
One day, Erden brings home a large infant dragon, which reminds Leticia of a feral cat she took to hospital in her past life. She manages to tame the beast by treating it gently, like a pet, and it becomes devoted to her.
