A dragon is a mythological beast, says the dictionary. I say dragons are real. There are many types of dragons. I won't speak of the different types here except to say they can be differentiated by color (red, green, blue, black, white) and by what they breathe out (fire, acid, lightning bolts, poisonous gas, frost.) Be wary if you meet up with a dragon and do not assume you can intimidate him by playing dead or acting crazy. That would be my advice.
A wyvern only has two legs. Two legs and a barbed tail. Since it has wings, bat-like wings, it really makes more sense that it would not have arms too. It has a barbed tail and a dragon's head. Or a head similar to a dragon's. I used to think that it is a Wyvern that appears on the Welsh flag until I became more attentive one time. No, your standard Welsh flag has a red dragon (Y Ddraig Goch) giving a friendly high-five. High-three, if you want to be more accurate.
Griffin, Griffen, Griffon:
More people than you might expect think these flying beasts are one and the same with just different vowels. It is a mistake to think this. A Griffin is primarily used in crossword puzzles, and the definition is always "fabulous animal." It has a lion's body and an eagle's head and wings.