The sound a horse makes instead of a neigh, at least according to Spongebob Squarepants.
Found in the episode My Pretty Seahorse, where Spongebob finds Mystery the seahorse.
Mystery: neighhhhhhh
Mr. Krabs: What was that?
Spongebob: Just practicing my impression of Mystery, Mr. Krabs! Weesnaw!
Everyone seems to associate "Weesnaw" with that Spongebob episode with the pet seahorse Mystery. But there's also a scene from an episode of an old Winnie the Pooh cartoon series with the word "Weesnaw". In that scene, Pooh and his friends are in the grocery store, when Eeyore tests out a ball horn that makes a loud "weesnaw" sound, and the grocery store clerk says: