CHAPTER DISCUSSIONS: DH7 - The Will of Albus Dumbledore

[Caius Marcius]

No: HPFGUIDX 179070

 10.	Is this patronus appropriate for Arthur Weasley from what we 
know
> about his character?  Is it a foregone conclusion that some Weasley
> will have a weasel for a patronus?  Wouldn't Percy make a better 
weasel?

JKR says that she likes weasles, and considers them to be unfairly 
maligned.  

http://www.jkrowling.com/textonly/en/extrastuff_view.cfm?id=7

Since Arthur is likewise unfairly maligned in his post at MOM, his 
Patronus (from JKR's point of view) is singularly apt.

  - CMC