"Cockroach" is a perfect example of folk etymology. Speakers prefer that longer words be "digestible", able to be broken up into smaller, comprehensible meaning-chunks, and so, when confronted with long words that they can't easily parse into multiple chunks, speakers will sometimes make interpretable chunks instead by changing a few of the sounds of the original word.
If you've ever heard an older person refer to "Old Timer's Disease" for "Alzheimer's Disease", that's folk etymology.
This is particularly common in loanwords, which, since they follow the morphology of another language, aren't able to be broken into meaning chunks by speakers of the borrowing language. This frequently leads speakers to create folk etymologies for these borrowings. Some famous examples:
Key West, from Spanish Cayo Hueso "Bone Key". It always bothered me that Key West wasn't the westernmost key when I looked at maps of Florida as a kid, and now I know why - it was never "West Key", it was always "Bone Key", and sure enough, Key West looks a lot like a bone on a map.
Crayfish/crawfish, from French crevice (kruh-VEESH). Borrowed from French, English speakers heard that the second part of the word sounded kinda like "fish", and saw that the thing lived in the water, so folk etymologizing a compound ending in "fish" was straightforward.
Cockroach, from Spanish cucaracha was renamed after cock "rooster" and roach/loach, a type of fish.
My favorite kind of folk eytmology is diachronic, that is, when speakers forget what a word or a piece of a word in their own language used to mean, and folk etymologize it instead. This can even result in meaning shifts for the word in question after the folk etymology.
For example, hangnail comes from Old English ang-nægel "anguishing spike", which originally meant a foot corn, which can push into the skin toward the bone like a painful spike. Well, English speakers forgot that "ang" meant the same thing as the "ang" in "anguish", and folk etymologized it to "hang + nail" instead.
But, once the form of the word had changed, the meaning shifted as well based on the new form of the word, and so now a hangnail is the little flap of skin that sometimes hangs around the edge of fingernails.
Other examples of this sort of folk etymology with resulting meaning shift include forlorn hope, from Dutch verloren hoop "lost troop/boarding party"; sandblind, from Old English samblind "half-blind" (note also that Samwise Gamgee's first name actually means "halfwit"); and shamefaced, from Old English schamfast "firm in modesty", with the older meaning of fast seen in "fast asleep" and "tie/make something fast", that is, "fixed in place".