well, it's not really that strange when you have a look at the etymology of the word
pesce is italian (and most likely derived from Latin) for fish, and the suffix arian is normally affix to words like this such as vegetarian, libertarian etc
Wiktionary says of the suffix arian:
A believer in something.
An advocate of something.
I don't find it strange @princessmewmew! I just find it surprising that some people I know have never heard the word #pescatarian! That's what I was trying to say. My brain froze when I was writing that comment. Sorry about that.