I am not poking at your english, you obviously are fluent. You probably speak with better grammar than most people that speak english as a first langauge.
After trying this on paper, and then deciding I could not possibly answer the question. I could only answer which man has an unknown pet. I looked at various versions of this riddle in hopes of finding out if my answer was correct. I saw the question asked in about five different ways.
I also saw mention of the question being unanswerable as stated. The only way the actual question can be answered is for assumptions to be made. First off you have to assume the fish is alive. You have to assume the fish is a pet, you have to assume the unknown beast (from the deduction) is the fish.
If you look at the Zebra puzzle on wikipedia, you see that you are told (in the discussion section) you must assume the question means to say something other than actually stated to be able to answer the actual question. To be able to justify working the problem out.
I have no doubt that given these clues you can figure out which man has an unknown animal under his roof. But to assume it is fish is incorrect. Unless the question is worded differently. The correct way to word the question would be something like "Which of the five house owners keeps fish as a pet."
It needs to mention one of the five men somehow, by saying house owner or which house or what color house... It needs to clarify. It must also state in the question the fish are pets and not food.
By simply asking who owns the fish, I could answer in a million ways and never be proved wrong or right. Anyone on the planet could have the fish. The guy in the sixth house could. Maybe the five on the start of the street all have fish in the freezer. It could be a fishing town for all I know.
Again I am not poking at your english. I am just fairly certain this question was meant to be a joke for students. I would say I am 98% sure...
Another way for me to be able to accept the answer is how a person words their answer. Now if someone said "the man with the unknown pet probably keeps fish at home." I could see that as being a fair answer.
But alas the question isn't asking who the probable person is. It doesn't ask for a percentage type of answer it asks for a definant answer. I know that showing a little chart and replacing the word unknown with fish is not a correct answer.
The animal "fish" is also key in this question. As it is both plural and singular, and it could be seen as food.
In the zebra version the same plural/sigular issue comes up. "I could say the zebra of africa are owned by no man." The zebra can mean all the zebras in africa. Not a single zebra on this one street with the five houses.
I hope I made sense with this.
Last but not least, having an animal living with you doesn't make it your pet. Working animals are not considered pets. This question is butchered and makes you assume too much for there to be a correct answer.
Basicly for the question to be answerable you would have to do a lot of rewording from the original.