First, a link to an tutorial. It`s really easy and fun to learn. With a bit of experience you will be able to create new creatures in almost no time.
http://dwarf.lendemaindeveille.com/index.php/Modding_guideIf you want to play an dragon copy&paste the following into DwarfFortress/raw/objects/entity somewhere. Then Gen a new world.
Beware as i haven`t tested playing as an dragon yet*. I only played selfmade creatures till now. You should backup your dwarf fortress.
[ENTITY:DRAGON_CIV]
[MOUNTAIN_SETTLEMENTS]
[CREATURE:DRAGON]
[INDIV_CONTROLLABLE]
[trANSLATION:ELF]
[BIOME_SUPPORT:ANY_WETLAND:2]
[BIOME_SUPPORT:ANY_DESERT:3]
[BIOME_SUPPORT:ANY_FOREST:1]
[BIOME_SUPPORT:MOUNTAIN:4]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:2]
[START_BIOME:MOUNTAIN]
[SELECT_SYMBOL:ALL:EVIL]
[SELECT_SYMBOL:ALL:VIOLENT]
[SELECT_SYMBOL:ALL:DEATH]
[SELECT_SYMBOL:ALL:UNTOWARD]
[CULL_SYMBOL:ALL:DOMESTIC]
[CULL_SYMBOL:ALL:FLOWERY]
[CULL_SYMBOL:ALL:HOLY]
[CULL_SYMBOL:ALL:PEACE]
[CULL_SYMBOL:ALL:NEGATOR]
[CULL_SYMBOL:ALL:GOOD]
[FRIENDLY_COLOR:7:0:0]
[DEFAULT_SITE_TYPE:CAVE]
[liKES_SITE:CAVE]
[TOLERATES_SITE:CAVE]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[MAX_STARTING_CIV_NUMBER:2]
[START_GROUP_NUMBER:1]
[MAX_POP_NUMBER:500]
[MAX_SITE_POP_NUMBER:20]
[ETHIC:KILL_ENTITY_MEMBER:ACCEPTABLE]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:ACCEPTABLE]
[ETHIC:TORTURE_FOR_INFORMATION:ACCEPTABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:ACCEPTABLE]
[ETHIC:TREASON:ACCEPTABLE]
[ETHIC:OATH_BREAKING:ACCEPTABLE]
[ETHIC:LYING:ACCEPTABLE]
[ETHIC:VANDALISM:ACCEPTABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:ACCEPTABLE]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_KILL:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:ACCEPTABLE]
You won`t be able to breath fire, though. Only Npc may breath fire.
*I don`t know how the game handles Megabeast on Playable creatures for example. However, whatever the result it should be
fun.