As I remember, 虎 and 鼠 are standalone nouns, meaning tiger and mouse, respectively. However, it seems that 老虎 and 老鼠 are more commonly used.
Why is 老 used as a prefix for 虎 and 鼠? Why do we use 獅子 instead of 老獅? Does it imply that tiger and mouse are old/experienced? What was the origin of such usage?
EDIT: Also refer to Aminopterin's answer and Travis Hu's answer for more insights.
After some research, I found two reasonable explanations. But, IMHO, the two should be compiled as the following:
老 is a prefix that is added to make 虎 and 鼠 easier to pronounce; besides, it implies that people respect 虎 and fear 鼠.
The two explanations as follow:
In the Classical Chinese era, texts were concise and some, if not most, of the words were one-character words. Yet, in colloquial context, people prepended/appended affixes to make the words easier to pronounce. These "new" words then became widely accepted and conventional.
Some common affixes and their associated words:
獅子、兔子、鴨子、猴子、鏡子、桌子、椅子、房子、筷子、刀子...
lion, rabbit, duck, monkey, mirror, table, chair, house, chopsticks, knife...
老虎、老鼠、老鷹、老師...
tiger, rat, hawk, teacher...
甜頭、苦頭、舌頭、骨頭、石頭、木頭...
sweetness, suffering, tongue, bone, stone, wood...
In Chinese, 老, apart from aged/experienced, has the implicit, figurative meaning of respected because people respect elderly so as to comply with the social value. That said, people prepended 老 on the ones they respect. This is why 老虎 and 老師 are named the way they are.
老鼠, on the other hand, has various interpretations. Some state that it fits to literal meaning of 老 – old. According to the Compendium of Materia Medica(本草綱目), a medical book, rats have the longest life span. Such misunderstanding came from the impression that rats have a high
rate of reproduction and have been around people back then for the longest time.
Some argue that the 老 in 老鼠 implies the cunning characteristics of rats. This interpretation can also be applied to 鷹. The rest argue that people chose 老 for 老鼠 to imply their fear toward rats and that rats are not easy to deal with.
As to why 獅 is named 獅子 but not 老獅, the reason is intuitive – to avoid conflicting pronunciation with 老師.
A bit late here, but @Thomas Hsieh, I think, was completely wrong (no offense).
In Old Chinese (from earliest attested times until 漢), all characters had a very complex pronunciation; they had diphthongs or triphthongs, two or three consonants each in the beginning and in end. (See: 複輔音)
Throughout Middle Chinese (from 魏晉 on until 元, a loose term though), most consonant clusters had been gradually lost. I guess this was because the Chinese Empire had expanded considerably and was constantly at war with foreign people. As a result, many 2nd-language learners arose; since they could not speak Old Chinese well, they simplified their pronunciation in a strange manner.
In the end, the native Chinese adopted simplified Chinese as well. However, many words had no longer been recognizable.
Imagine:
When in the Course of human events it becomes necessary…
becoming:
Wa ee da ko oo hoo ma ee va ee bee ko na see sa ree…
This is indecipherable, right? To fix the situation, meaningless suffixes were added. "Well you said “鼠” a rat — or did you mean “數” to count, or “屬” to belong to, or “暑” a hot and humid day?" Hence, we agreed on “老鼠” for mouse, “計算” for to count, “屬於” for to belong to, and “大熱天” for a hot and humid days to remove the ambiguity.
You can see:
In the beginning “老” was a marker for reverence, but in “老鼠”, it seemed to have been borrowed to be a marker purely for disambiguity, as you do not normally revere a rat. Similarly “子” was a diminutive ending (compare “孩子”, “小子”, also English “-y” as in “Kitty”, or “-et” in “nymphet”), but it was borrowed in “獅子” purely for disambiguity — when you say “獅子” you do not mean it to be cute!
I found something: 汉语词缀的性质与汉语词法特点 (loosely: Properties of Chinese prefixes and suffixes, and features of their word-forming rules)
佬. It struck me to be very curious that whether 佬 and 老 are cognate! Shall leave that to linguist though.
Commented
Dec 1, 2016 at 5:48
Seems that you have had the right answer (and you even asked your own question). But your question does stimulate my curiosity. And I search some resources from 漢語大詞典 and Web. Here are what I have found, hoping these information would help.
老
前綴。放在某些指人或動植物的名詞前，構成名詞。
▶ 唐·寒山《詩》之二六八：“老鼠入飯甕，雖飽難出頭。”如：老虎、老玉米、老百姓。
子
名詞後綴。
▶ 《宋書•朱齡石傳》：“齡石使舅臥於聽事一頭，剪紙一方寸，帖着舅枕，自以刀子懸擲之。”
▶ 唐·張鷟《游仙窟》：“千嬌眼子，天上失其流星；一搦腰支，洛浦愧其迴雪。”
▶ 宋·羅大經《鶴林玉露》卷十：“誠齋曰：‘相公且子細，秀才子口頭言語，豈可便信！’雍公大笑。”
▶ 清·黃宗羲《馬雪航詩序》：“程子言性即理，差為近之。然當其澄然，在中，滿腔子皆惻隱之心。”
▶ 張天翼《春風》：“這麼着他細細裏嚼着，臉子微微地側着，好像在那裏欣賞自己那種文雅的嚼聲。”
某些量詞的後綴。
▶ 《朱子語類》卷一•四：“今日看《中庸》，只看一段子。”
▶ 劉半農《麵包與鹽》：“一個錋子的鹽，擱上半喇子兒的大蔥。”如：兩下子，那陣子；幾幫子；一輩子。
用於動詞、形容詞詞素之後，構成名詞或名詞性詞組。
▶ 《封神演義》第十二回：“夫人曰：‘我兒，你往哪裏耍子，便去這半日？’”如：胖子；墊子；矮子。
The followings are my understandings after reading some valuable answers:
When we add prefix 「老」 before animal nouns, we want to show the respect and reverence. So your example 「老虎」「老鼠」 could be explained.
You may wonder why Chinese people add 「老」 before「鼠」: Shouldn't this animal be bad because mice always stole?
In Ancient China, people treat mouse as a holy animal because mouse is smart and agile, and it has strong reproductive capacity. Some clans even see mice as totem. And this could explain why Mouse is the first on Chinese Zodiac.
As for 「獅子」, an explain is that 「獅子」is a foreign words. This animal doesn't originate from China.
In a word, when you try to comprehend the definition of a word or a phrase, you'd better think as the way Old Chinese did, because there are huge differences between Old Chinese and Modern Chinese.
I wish to leave comment instead, but I am new user, so...
Some Chinese words @Thomas Hsieh mentioned in answer and comment are wrong.
First, I'm pretty sure 鼠佬 and 虎佬 are not used in Mandarin Chinese, Cantonese, and Written Chinese. I can't get any meaning from them.
Second, 甜頭 does not mean sweetness. It means benefit (好處), the opposite of 苦頭.
To be honest, I never realise 老虎 and 老鼠 have the same prefix. I can't think of any reason why 老虎 and 老鼠 have 老 as prefix.
PS I am native Cantonese speaker.
佬as a suffix is used in Cantonese only? as in大佬,鼠佬,虎佬,鬼佬.