How would you say "X, the Y" in Chinese? I am currently learning Mandarin in Simplified Chinese. I've just started learning so I cannot speak all that well. However, here is my problem:

I have a Chinese family coming over. They are proud that I am learning so I am making a greeting for them in Chinese. I also happen to have a pet moth, named "Seven". I think in Chinese the kanji for moth is 蛾. To put it simply, I want to write "Seven the Moth" on its tank and am I confused now.

One way is literally 七蛾 (seven moth), like "moth" is the title of Seven, like a name with Mr. or Miss or the title at the end. But then, they may think there are seven moths.

So, I think for another way using "de" like 蛾的七 (moth de seven), but I don't think this is correct, as it refers to an adjective modifying a noun.

The last way I can think of is like the first but reversed: 蛾七 (moth seven) which sounds the most correct, but I have no idea what to write.

What should I write on the tank?
Domestic animals like dog, cat, horse, cow, pig, goat, chicken and duck are often described with a single syllable word in Chinese:

狗，猫，马，牛，猪，羊，鸡，鸭

There are few exceptions, For example, snake is not a domestic animal, but it is still being called with the single syllable word 蛇

For other animals, it is customary to use two syllables words to refer them. 

For example: 

Tiger (虎), is always called 老虎

Elephant (象) is always called 大象

Monkey (猴) is always called 猴子

Frog (蛙) is always called 青蛙

In the case of moth(蛾), it is always called 飛蛾

So, your should refer your pet moth as  '飛蛾'

Calling someone by a single syllable name is rare because it sounded blunt. Most of the time we would add title or a name prefix (indicates a noun is a name) to make it a two syllable name.

For example: 

The name '七' would be '小七, 阿七 or 七仔' in day-to-day conversation

In your case, "my pet-- Seven the moth" should be translated as '我的宠物--  飛蛾小七 (or '阿七)' 

Or you can say: " 小七, 我养的飛蛾" (little Seven, the pet moth I keep)

Edit: one more suggestion: 

Since you called Chinese character as kanji, here is another suggestion-- 'Seven' in Japanese is なな (nana). You could call it '娜娜(Nana), which is a common female pet name.