Depends if you want to describe the fur itself (horripilated) or the fluffed up cat (horripilating). The "was" might be extraneous in the first example.

"Horripilate", as in getting goosebumps when exposed to fear, cold, and whatnot. An example in a sentence would be: I suddenly horripilate when I watch a violent movie. … I horripilate when I see something scary. (MORE)