...either you're missing the "anthro" or "human" tag, or you slipped away from ponies temporarily.

His eyes were locked onto her breast with utter, terrified rapture.

While it is useful as setting the tone that something might be wrong with Noteworthy, it does not makes sense if they are regular ponies, as it would mean that Applejack wouldn't have any breasts that could be easily stared. Especially if she was facing the pony she was talking to.

7615914
Not quite.

For humans, breast and breasts are synonymous. That's not true for many mammals who have their breast bone facing forward, while having one of more sets of breasts on their stomach or abdomen. Wolves, for instance, have a forward-facing breast bone, vertical instead of horizontal ribs, and have somewhere between eight to ten teats(breasts) depending on the breed. Horses have the same, except two teats(breasts) between their hindlegs.

I wasn't speaking about breasts in this context, only the breastbone. The breast. That is why I left off the 's' and had it remain singular. If I was speaking about breasts/teats, it doesn't make sense to use the singular because having only one be seen is such a horribly pedantic detail that it might as well be excluded in editing.