Did you know Hapkido is written 合気道 in Chinese letters? Which, we Japanese read, yes, you guessed it, Aikido.
The founder of Hapkido learned in Japan 大東流合気柔術, under the same teacher who taught the founder of Aikido. So it's natural there are a lot of similarities between the two.
To be precise, Koreans would write 合氣道. But 気 is only a simplified form of 氣.
In many Japanese martial art systems including Karate, they call yelling
kiai. This is written 気合. Notice these two letters are the same first two letters of Aikido/Hapkido, just in the reverse order. Koreans use exactly the same word; they just read it
kihap.