Takahashi Rumiko Gekijou Ningyo no Mori

Both shows involve 2 main characters who are immortal and they want to become normal humans. If you liked 3x3 Eyes chances are you will like Mermaid Forest or vice-versa. 

In Hi no Tori, eternal life is given to anyone who drinks the blood of the phoenix. In Mermaid Forest, eternal life is given to those who eat mermaid flesh. For various reasons, people search for these mythical creatures to obtain such a gift. However, others endure it as a form of punishment and wish to become normal human beings again. If you enjoyed one of these anime, then you might like the other. 

To me, they are similar if we're talking about immortality. They are similar yet different. Mermaid Forest focus on immortal ones who wants to be normal again. But FMA Brotherhood focus on normal beings who want immortality. Desperately finding a way to each road they take. 

To be honest I started watching Mermaid Forest for 2 reasons: the opening and because I thought it'd be like Nightwalker but with mermaids and I wasn't wrong. They share the same genres: mystery, horror, fantasy, drama (even if Nightwalker is not categorized as fantasy and drama, it is). The pace in both shows (episodic) and the dynamics on every chapter are exactly the same. They both deal with immortal main characters. This is personal but I had the same routine watching them (one episode per day or two if they are double-episode) and enjoyed them the same way cause they have more or less the same chill vibe. Another personal taste here: they both have excellent opening themes with excellent artist (Chiaki and Buck-Tick) but kinda shitty and forgettable endings but they grew on me (I actually ended liking Mayonaka's ending lol). Well, I really recommend Mayonaka to anyone who wants to just sit back and relax. I personally like more NightWalker than Mermaid. read more 

Did you find the concept of eating a mermaid's flesh and becoming immortal fascinating? Both of these animes have a protagonist that did just that. On top of that, both series feature various other creatures from Japanese mythology and mysteries. They both also have a male/female partner protagonists, with more or less romantic tension between them. 

Protagonists on a journey trying to find something they desire to change who they are. In InuYasha's case, he wants to become a complete youkai. In Mermaid Forest, Yuta wants to become a normal human. In the romance aspect, the protagonists are also really similar. Both had trouble falling in love for women who weren't immortal like them, and both series show a bit of how they dealt with it. Furthermore, they're by the same author, Rumiko Takahashi, and you can really feel her presence in the works.  

Both stories are a bit morbid and dark. Pet shop of horrors contains more light humor but there similar in both art and the darkness of creatures. 

Searching for people who keep coming back to life just like in Ajin? Well, Mermaid Forest is just like that. In Mermaid Forest, those who eat mermaid flesh become immortal. The protagonist, who is 500 year old, search for a way to turn back into a normal human. The plot may not sound so similar to Ajin, but whenever some character dies and comes back to life I simply can't think about anything else. It really gives a lot of Ajin vibes.