Today I had two weird happening on the roads in Skyrim. At first I was walking on the roads at night time near Solitude and saw a ghost horse. I followed it as long as I was able but lost it a while after Dragon Bridge. Now I was travelling on somewhere on roads in wilderness when I saw a lonely cow which was full of weird painted markings. I followed it long time but near of Whiterun it started to run faster than bunnies can and I lost it.
Has anyone else seen those creatures? And if you have can you tell me anything else about them? I'm really curious to get know everything which is connected to them.

Was there a headless horseman on top of that horse? If so follow him and he will lead you to an area. As for the painted cow, there is a side quest where a farmer has lost his prized goat/cow, and a giant has taken it. That was the only time I saw a painted animal of that kind.

The cow I believe is a sacrificial cow being offered to giants by a farmer. Maybe the farmer met an untimely end but the cow got away? Don't know about the horse. I'd like to see that.

No there was no rider on it and it ran too fast to be followed long time without horse. When the horse ran next to me there was "steal horse" command when I looked at it but nothing happened when I tried it.
I think that the cow I saw may be from that quest you mentioned but if it's so I can't understand why it's wandering alone on the roads.

Even before accepting the quest and speaking with the farmer early in the game I saw no prized animal. After a while and still accepting the quest I found this weirdly marked cow wandering alone. I had run into the farmer and he mentioned a lost cow, then when I went to the marker there was a giant by the cow, they probably just spawn a giant companion with the cow once the quest is activated.
I was quite perplexed myself finding a painted cow at first, I recognized the painting from the giant camps and assumed some poor giant didn't have a mammoth and lost his cow lol

I will have to find more info on the ghost horse, never encountered just the horse before.

EDIT: From what ive been reading on forums, that is the headless horsemans horse. Eventually the horseman should catch up or even just appear on his horse. Next time just try to keep up with it. It's an easter egg thrown in by Bethesda.