In fact, although the words merged through linguistic drift (totally normal process), both emoticon and emoji have similar vintage and originally described different things. Emoji came from Japanese, and originally was coined as "kaoji," or "face characters." They were designed to be a convenience for Japanese phones so some things could be sent in fewer SMS character-limited messages by including a face. This shifted to emoji because they were used mostly to indicate emotions. Emoticon came from the early internet and thus the US, and primarily referred to a practice of using ASCII characters to make faces for a similar purpose.
As the internet grew and phones gained more functions, kaoji gained a meaning much like emoticon had in the USA, but primarily the Japanese style faces focused on the eyes for expressions (see: ^^ -- owo n_n etc.) while USA versions focused on the mouth as in :) :( :P etc. However, emoji also moved from literally being part of a font for phones and started being implemented as images, and they proliferated out from a standard set into multitudes. This style of para-linguistic emotive communication was perfect to jump across language barriers, and so through cultural interfaces such as anime fans, online gaming, and the like, the use of the term emoji leaked into English, and due to similarity, eventually came to be used as a catch all term for everything described here as it spread out in use through social groups mostly to entirely unfamiliar with the terms and their origins.
But hey, that's language. It's normal for people to do that with language. It's cool kids, they're your words now too. But hey, learning the history of words is fun and interesting. I might be biased though, I did go to school for that.
