OK, say you want to make a Santa Clause creature; it would have the same body as a human, except that it might have this stuff, which you can just put in the body_default.txt. Body modding is fairly complex, so you should wait for that until you get the hang of creature, entity, item, and matgloss modding.
[BODY:COAT_RED]
[BP:RCOAT:big red coat][CONTYPE:UPPERBODY]
[BODY:HAT_JOLLY]
[BP:JHAT:jolly red hat][CONTYPE:HEAD]
[BODYGLOSS:BOOT:foot:boot]
[BODYGLOSS:MITTEN:hand:mitten]
Would replace the old creature entry body data as thus:
[BODY:HUMANOID:COAT_RED:HAT_JOLLY:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:5FINGERS:5TOES:MOUTH]
[BODYGLOSS:BOOT]
[BODYGLOSS:MITTEN]
The creature entry would be titled [CREATURE:SANTA_CLAUSE]. You can just copy humans and change the [CREATURE] tag. It is important that it is in all caps. In creature entries, only names (in this case [NAME:Santa Clause:there's only one Santa:clausish] being the name, plural form, and adjective form) and the [PREFSTRING:x] tags can be in lower case or have spaces. Change the entry in the [PREFSTING] tag to "jolly gut." This will make it so that you will occasionally see "Urist McChild likes there's only one Santa for their jolly gut.
You can change [FAT:3] to [FAT:7] which will make him give more fat when butchered and make him more resistant to cold. This can also be done by increasing the number on the [LAYERING:X] tag.
The [TILE:'U'] can be changed to [TILE:'S']. Changing [COLOR:3:0:0] to [COLOR:4:0:1] will make his tile bright red instead of cyan. Color combinations can be found on the wiki by searching "color." If you use a letter for a tile instead of a four digit tile code listed on the wiki, you need to put an apostrophe on either side. I made an "invisible" race once because I didn't know that, and it was hard to play with them.
Change [BENIGN] to [GOOD]; this will make it show up only in good areas. If you don't like Santa, you can make it [EVIL] instead, which functions similarly to [GOOD], only that it makes him show up in evil areas.
Since it isn't a creature that we are sure exists in the real world, remove the [MUNDANE] tag, unless of course you really believe in Santa. To make him only move around at night, change [DIURNAL] to [NOCTURNAL]. If you feel that the 60 to 120 years is too short or long a life for a Santa, change the numbers of the [MAXAGE:X:Y] to the range you want the game to use when randomly generating his lifespan. For example, to make it so that he lives between 1000 and 1500 years, enter [MAXAGE:1000:1500]
If you don't want him picking up and using items and weapons, remove the [EQUIPS] tag.
Of course, for him to actually show up, you need to add a biome tag, such as [BIOME:GLACIER], or you can add [MEGABEAST] or [SEMIMEGABEAST], which will make it so that he will attack your fort and you will occasionly find quests to kill him in adventure mode.
For your creature entry, you should get something like this:
[CREATURE:SANTA_CLAUSE]
[NAME:Santa Clause:there's only one Santa:clausish]
[TILE:'S'][COLOR:3:0:0]
[GENPOWER:3]
[INTELLIGENT]
[CANOPENDOORS]
[BENIGN]
[PREFSTRING:jolly gut]
[BODY:HUMANOID:COAT_RED:HAT_JOLLY:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:5FINGERS:5TOES:MOUTH]
[BODYGLOSS:BOOT]
[BODYGLOSS:MITTEN]
[SIZE:7]
[MAXAGE:1000:1500]
[ATTACK:MAIN:BYTYPE:GRASP:punch:punches:1:2:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:SECOND:BYTYPE:MOUTH:bite:bites:1:1:GORE][ATTACKFLAG_CANLATCH]
[CHILD:12][BABY:1][MULTIPLE_LITTER_RARE]
[FAT:7]
[EQUIPS]
[NOCTURNAL]
[BIOME:GLACIER]
[SPEECH_MALE:human_male.txt]
[STANDARD_FLESH]
[HOMEOTHERM:10067]
[LAYERING:70]
[SWIMS_LEARNED][SWIM_SPEED:2500]
[MUNDANE]
That's all folks! (for now; you should have a basic understanding of creature modding at this point, but there is more advanced stuff that can be done)
Oh, and Happy Holidays!