Well, first to address this "bleating of the deer" business. In English, we make animal noises by approximating them with letters. Dogs go "woof woof", cats go "meow meow", etc. In Chinese, they approximate with characters. 吼 is the sound of a roaring lion. A cat meow is 喵喵.  Sheep go 咩. And apparently, 呦 is the sound a Chinese deer makes. 

In this case, obviously, we're not literally talking about the bleating of a deer, but we are perhaps talking about bleating in general. In English, when we make an exclamatory noise in a sentence, it often (but not always) goes at the beginning of the sentence, like this, "Woah, that was crazy!" or "Yikes! I almost stepped on that." "Gosh, I'm so embarrassed." In Chinese, they have similar ways to enhance a sentence's meaning. One way to do that is to add a sound to the end of a sentences that makes it stronger or acts as a "yikes", a "woah", a "gosh", etc. - in this case, they're using 呦.  There are lots of different characters we can add to the end of the sentence that are, in essence, just sounds, but that strengthen the meaning of the sentence or can even change it a little. 

呦 is really just a sound being used as a general exclamation, but also, if you google 后悔呦 by itself, you'll see that this 呦 is very often used with 后悔.    

Example: 租不租？别后悔呦！(To rent or not to rent? Don't do something you'll regret!)

So, I'd read that as, "But gosh, won't you regret it?"

Here's another site with some more examples of 呦

<a href="http://www.chinaorb.com/index.php?s_word=%E5%91%A6" rel="nofollow">http://www.chinaorb.com/index.php?s_word=%E5%91%A6</a>