It's definitely a quirk of the English language that "octopus" can sound like it should be a sentence ender, even though it's not grammatically correct. It's probably because the "us" ending makes it sound like a plural, and we often end sentences with plurals.
Whether it's annoying or not depends on the individual! Some people might find it mildly irritating, while others might just find it amusing. It's all about personal preference!
Do you find it annoying?