Skeleton wolves?

Today I moved my old worlds into the saves folder and take screenshots. And I also found an empty resource pack I made. But when I open one of my old worlds, I saw two tamed wolves that looked like skeleton wolves. I tried to spawn a another wolf, it looked like normal. But whenever I tame it,it looked weird again. I thought it was effected by biome , so I opened a another world, it still looked weird. Then I tried to change my resource pack back, the game crashed without a crash report.

This is the second time I've seen this. I'm not sure this is a bug, it could be an Easter egg. I didn't install any mod. My computer is always buggy when playing minecraft.

Sorry for my bad English.