In Young Frankenstein why do the horses whinny at the mention of the housekeepers name?

Because it's funny

Blucher Young Frankenstein

The real reason:

Every time a character speaks the housekeeper's name, nearby horses whinny and neigh. A generation of film-goers, missing the obvious humor behind the gag, are now convinced they know the subtle "secret" that makes this "in-joke" funny: "Blucher" is the German (or Yiddish) word for "glue," and so the horses react in distress whenever they hear the name (because glue is made from horses, ya know).

The only in-joke connected with this gag is that so many people have missed its real humor and instead been taken in by a leg-pull. "Blucher" is not the German word for "glue," nor does it sound remotely like any German word for "glue" — standard, slang, archaic, or otherwise. Blucher (or Blücher) is simply an ordinary Germanic surname. The joke employed in the film is a take-off on the hoary melodramatic film device of inserting an ominous organ riff or clap of thunder and having actors react with visible fright whenever the villain appears on-screen or a character refers to something evil or threatening. (Indeed, an ominous flash of lightning and clap of thunder accompanies Friedrich's first encounters with both Igor and Frau Blucher.) The "horse" bit is funny because the Frau Blucher character is neither evil nor a villain (but a disagreeable-looking harridan), because the horses are scared by the mere mention of Frau Blucher's name but are undisturbed by her physical presence, and because the gag is used ridiculously often throughout Young Frankenstein. (Horses are heard whinnying at any repetition of the words "Frau Blucher," even in settings — such as the interior of Frankenstein's castle — where no horses are present!) The second point is what seems to have thrown many viewers: The horses are terrified at hearing the name "Frau Blucher," but Frau Blucher herself doesn't bother them at all. Apparently some people have taken this bit of humor too literally and assumed the joke must have something to do with the name itself.

For some of us, the desire to believe we've been let in on a piece of secret knowledge can overpower our good sense and lead us to ludicrously arcane readings of the straightforward. Full appreciation of Young Frankenstein's broad humor requires only a familiarity with some common cinematic devices, not the process by which glue is made.

Young Frankenstein Frau Blücher

Because shes evil and the horses know this and just the sound of her name gives them the whinny's!

I'm making a joke I know it's really willy's.

She's supposed to be such a frightening person that the mere mention of her name terrifies the horses.

No. Blucher is NOT German for glue. Kleber is German for glue. The horses reaction is more of a spoof on the melodramatic organ riff and scared reaction of actors on other old horror movies. The "glue" theory is urban legend. PLEASE click the link.

I think her name means glue factory and back in the days that is where horses ended up in glue factories

Frau Bleaucher--Bleaucher means GLUE so horses don't like that!!

hahaha think about it some more cause I ain't telling!

or you could google Catherine the Great, horse