Hm... I wouldn't worry. She is just a healthy little girl, many children get scared of things like that. Little phases. I have an 8 year old cousin, who was once scared of dogs, but he got over it. When I was 2 or 3, I was scared of a video game character. Got over it in time.Now, I laugh about it. Just be a good mother, reasure her, and she'll get over it. Simple fix. :)
Listen to the song Demonslayer by voltaire on youtube. The father's advice is good overall.
Also maybe give her a plastic sword to wap him with if she sees him at night? Or set her stuffed animals up around her bed to "guard" her. This can sometimes give children the security they need. Just out of curiousity what was the cartoon?