In construct state (usually) only the first noun is pluralized.
Birthday - יום הולדת, birthdays - ימי הולדת
Pet - חיית מחמד, pets - חיות מחמד
This is different from noun+adjective, in which both the noun and the adjective get a plural suffix:
A hot day - יום חם, hot days - ימים חמים
The word מחמד is rarely used outside the idiom חיות מחמד. A friendly animal is חיה חברותית or חיה נחמדה or חיה ידידותית. But there is also a grammar point here: the difference between a construct state and a noun + adjective combination. Note how the ה in חיה becomes ת in חית מחמד, this is because it's a construct. As radagastthebrown explained, when pluralising a construct, only the first word changes, whereas "friendly animals" are חיות נחמדות. And adding a definite article, in the construct, only the second word gets an article - חיות המחמד, but החיות הנחמדות.
Could someone help me out on the exact translation of these two words? "חיות מחמד"
חיות = animals
מחמד : There is no translation for this word. Broadly it's the feeling you have towards pets or children
So it loosely translates as "animals of machmad"
It's more like that מחמד is the Hebrew term for endearment. So animals of endearment.
It's tough to give an exact definition because the word is hardly ever used outside of this expression. Wiktionary defines it as a "pleasant or lovely thing"
מחמד is also used in a slang for an old hi-tech worker for example, as a זקן מחמד
I have never heard of זקן מחמד, but it sounds similar to ערבי מחמד or דתי מחמד, which are rather disrespectful.
@radagastthebrown 1. Strange that I can't response to a person who have responded to me, but anyway: 2. That fact that you have never heard this specific idiom/expression, doesn't necessarily means it doesn't exist, as I said = it's a hi-tech slang, and people who I have heard this expression from and friends of mine who work in other companies do know this expression. 3. Slang usually is not about respect someone. And even than - onone has to take it on their part. 4. Hebrew currently contains about 75-80K words, while most of peoples' vocabulary on a daily basis is about (English) ~5K, so I am guessing there are a lot's of words you and me have never heard before. :) BR,