Where did the phrase, 太牛了 come from?
I'm guessing it doesn't have anything to do with cows...
It came from 牛逼. 牛逼 means cow's genital orginally and it is derivatized into impressive due to it's wide. Many people are saying it even it's bad meaning. You can both use 太牛了 and 太牛逼了 though.
At first, it was 吹牛皮, which means blowing up the cow skin. 吹 means blow up and 牛皮 means cow skin.
Long ago, butchers blow up the pig skin to shave hairs. However, a cow is much bigger than a pig. So, when someone says he/she is able to blow up the cow skin, he/she must be talking big (吹牛皮).
Sometimes，speaking rudely sounds cool or more impressive. So, 吹牛皮 turns to 吹牛逼. 牛逼 means cows' vagina. We use 逼 but not the correct Chinese character, because 牛逼 sounds more acceptable. It's just like shot and shit.
吹牛逼 means talking big, while 牛逼 means "real big". 牛 is short for 牛逼.
Check the link below. I got most of my opinions from it. My English is not good. If there is any mistake in spelling or grammar, just correct it.
Cows are very stubborn and stupid kind of animal while having unique personality, so when use them to describe a person, we can assume that person has done something very awesome that a normal logical person will not even try. Thus, very cow like. (太牛了)
If that person try to do something that a normal logical person will not even try and failed, then we can call him "太猪了". (Very pig like or stupid)
太鼠了 （ Very rat like = cowardly ) 母老虎 (Female tiger-tigress) If your wife is a female tiger, she is fierce and will hit you and make you scare of her.
Chinese use animals as an adjective in this way so we don't sound too aggressive while being humorous.