Without the name of a different animal mentioned before 肉 (such as 孜然羊肉), does the word 肉 always mean "pork" without any exception?
I have found the following counterexample:
See L21 below; 肉丝 could be beef or pork.
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I worked in restaurants for many years. As I understand it, '肉' in Chinese menus only refers to pork unless it is specifically stated what kind of meat it is.
時菜炒（豬）肉片 = in-season vegetable stir-fry pork slice
時菜炒牛肉 = in-season vegetable stir-fry beef
（豬）肉絲湯麵 = shredded pork noodle soup
雞絲湯麵 = shredded chicken meat noodle soup
(豬) is omitted most of the time because when no specific is given, it is the default meat.
But there are always exceptions:
Since the most common meat you see in 夹馍 are 羊肉 (mutton) or 牛肉 (beef), even when people reduce 羊肉夹馍 or 牛肉夹馍 to '肉夹馍', '肉' still refers to mutton or beef
If it is 清真食物 (Halal food) menus, you can be sure 肉 on the menus are not pork but mutton or beef instead
Outside of menus, 肉 can refer to any kind of meat, for example: 肉 in "大魚大肉" or "有酒有肉" does not refer to a specific meat.
In Standard Chinese, 肉 means any kinds of meat, while 猪肉 means pork. This is technically correct in the standard expression.
However, in colloquial Chinese, unprefixed 肉 usually means pork, except in the “halal menu”. It is true in 99.9% of the situation.
If you are writing legal documents or contracts, use 猪肉 to refer to pork. Otherwise, just use unprefixed 肉 for pork.
For Hans culture(in fact, there is no such things as Hans ethnics) , 肉 usually means pork. If you look at the glphy 家 （family), the script below is a PIG 豕, origin from the oracle glyph. Because all food waste can be used to feed pigs.
Historically, cow is important to agriculture, there are era that forbids people from eating cow and death penalty is imposed (to prevent people stealing cow and slaughtering cow with various excuses) .
Herding tribe in the Northen China don't rear pig, they either eat goats/sheeps, cows or horses. When adapting the Hans culture glyph, those culture will explicit specify the type of meat they are eating in mandarin language.
However, depends on popularity of particular food, when time pass, people simply omitted the type of meat name. e.g. 兰州拉面 original called 兰州牛肉拉面. Because it is a well know beef noodles dishes, everybody knows you will not find pork inside 兰州拉面.
I would say 99% of time it is true. However, it is always better to verify with the waitress if you're not certain. For example:
肉捲 - meat roll 碎肉 - mince meat
While mostly are pork and does not have a different animal mentioned, it could be other meat such as beef / chicken.