This one is more for you skull guys!!

[D.C.O]

I had a customer bring in a cow skull.....Ok that's normal but then he explanes he told all the kids in his neighborhood he found a DINASORE HEAD!! LOL Yes you know whats coming next he wants me to turn this cow skull into a Dino skull!! ANY IDEAS

[Toxic]

first you need to clean it then stain it so it looks aged, if its not clean it would not be good to handle it (kids) and the smell would be bad. other folks here have used coffe and tea to do some really good staining that makes it look old.  Adding anything like horns and hogs teeth to it would be on you.

[PLTannery]

Apoxie Sculpt some lumps, Bumps, Horns and some Fancy teeth... should be able to make a right proper Dino. and Then paint it antiqued to finish the effect and cover the color difference..

[Alpinist]

I haven't seen many fake dinosaur skulls, but fake dragon skulls are pretty common and might give you some ideas. Search the deviantart.com web site for "dragon skull". Here are some good ones, though they are not made from cow skulls:
http://zhon.deviantart.com/art/Cragback-Dragon-Skull-74823494
http://zhon.deviantart.com/gallery/744916#/d1bwj8v
http://reptanglian.deviantart.com/gallery/?q=skull#/d3rf2af

This one looks the most like a real dinosaur skull:
http://zoroha.deviantart.com/gallery/?q=skull#/d2xg0i5

A Google image search for "dinosaur skull" will show you a variety of real ones, and a few fakes.

[D.C.O]

Thanks guys!

[The Dog]

ha, awesome post some pics when you get it done. 

[Sea Wolf]

Apoxie Sculpt some lumps, Bumps, Horns and some Fancy teeth... should be able to make a right proper Dino. and Then paint it antiqued to finish the effect and cover the color difference..

Clean it good and then fancy it up as above. Great for Halloween too.

[Baccus]

This one looks the most like a real dinosaur skull:
http://zoroha.deviantart.com/gallery/?q=skull#/d2xg0i5

That last pic is a dinosaur skull.  Dracorex hogwartsia is actually named after the school in harry potter and everything, but it is a real dinosaur.  The current theory is that it is a younger form of one of my favorites, pachycephalosaurus. 

In terms of turning a cow into a dinosaur, adding a lot of texture and weight is a good way to go.  Remember that fossil is basically stone in the shape of bones.  I have seen all sorts of colors in fossil as well.  Everything from your standard brown and blacks all the way to nearly opal patterns and I have a few fools gold ammonites as well. 

Post pics of what you decide to do.