Code taken from https://gist.github.com/warmist/8563110. Stripped out anything not important for creating a simple wild creature

make_creature.lua

args={...}


function gen_attribute(array)
    local a=math.random(0,#array-2)
    return math.random(array[a],array[a+1])
end


function get_body_size(caste,time)
    --TODO: real body size...
    return caste.body_size_1[#caste.body_size_1-1] --returns last body size
end


function gen_body_modifier(body_app_mod)
    local a=math.random(0,#body_app_mod.ranges-2)
    return math.random(body_app_mod.ranges[a],body_app_mod.ranges[a+1])
end


function find_race(name)
    for k,v in pairs(df.global.world.raws.creatures.all) do
        if v.creature_id==name then
            return k
        end
    end
    qerror("Race:"..name.." not found!")
end


function get_caste(race_id, caste_id)
    local cr=df.creature_raw.find(race_id)
    return cr.caste[caste_id]
end


function create_creature(race, caste_id)
    local race_id = find_race(race)
    local unit=df.unit:new()
    local caste=get_caste(race_id, caste_id)
    unit:assign{
        race=race_id,
        caste=caste_id,
        sex=caste.gender
    }

    if unit.flags1.tame == true then unit.flags1.tame=false end

    local pos=copyall(df.global.cursor)
    if pos.x==-30000 then error("Place cursor somewhere") end
    unit.pos:assign(pos)

    local body=unit.body
    body.body_plan=caste.body_info
    local body_part_count=#body.body_plan.body_parts
    local layer_count=#body.body_plan.layer_part
    --[[ body components ]]
    local cp=body.components
    cp.body_part_status:resize(body_part_count)
    cp.numbered_masks:resize(#body.body_plan.numbered_masks)
    for num,v in ipairs(body.body_plan.numbered_masks) do
        cp.numbered_masks[num]=v
    end
    cp.layer_status:resize(layer_count)
    cp.layer_wound_area:resize(layer_count)
    cp.layer_cut_fraction:resize(layer_count)
    cp.layer_dent_fraction:resize(layer_count)
    cp.layer_effect_fraction:resize(layer_count)
    
    local attrs=caste.attributes
    for k,v in pairs(attrs.phys_att_range) do
        local max_percent=attrs.phys_att_cap_perc[k]/100
        local cvalue=gen_attribute(v)
        unit.body.physical_attrs[k]={value=cvalue,max_value=cvalue*max_percent}
    end

    
    body.blood_max=get_body_size(caste,0) --TODO normal values
    body.blood_count=body.blood_max
    body.infection_level=0
    unit.status2.body_part_temperature:resize(body_part_count)
    for k,v in pairs(unit.status2.body_part_temperature) do
        unit.status2.body_part_temperature[k]={new=true,whole=10067,fraction=0}
        
    end
    --[[ largely unknown stuff ]]
    local stuff=unit.enemy
    stuff.body_part_878:resize(body_part_count) -- all = 3
    stuff.body_part_888:resize(body_part_count) -- all = 3
    stuff.body_part_relsize:resize(body_part_count) -- all =0
    
    stuff.were_race=race_id
    stuff.were_caste=caste_id
    stuff.normal_race=race_id
    stuff.normal_caste=caste_id
    stuff.body_part_8a8:resize(body_part_count) -- all = 1
    stuff.body_part_base_ins:resize(body_part_count) 
    stuff.body_part_clothing_ins:resize(body_part_count) 
    stuff.body_part_8d8:resize(body_part_count)
    
    --TODO add correct sizes. (calculate from age)
    local size=caste.body_size_2[#caste.body_size_2-1]
    body.size_info.size_cur=size
    body.size_info.size_base=size
    body.size_info.area_cur=math.pow(size,0.666)
    body.size_info.area_base=math.pow(size,0.666)
    body.size_info.area_cur=math.pow(size*10000,0.333)
    body.size_info.area_base=math.pow(size*10000,0.333)
    
    unit.recuperation.healing_rate:resize(layer_count) 
    
    --appearance
    local app=unit.appearance
    app.body_modifiers:resize(#caste.body_appearance_modifiers) --3
    for k,v in pairs(app.body_modifiers) do
        app.body_modifiers[k]=gen_body_modifier(caste.body_appearance_modifiers[k])
    end
    app.bp_modifiers:resize(#caste.bp_appearance.modifier_idx) --0
    for k,v in pairs(app.bp_modifiers) do
        app.bp_modifiers[k]=gen_body_modifier(caste.bp_appearance.modifiers[caste.bp_appearance.modifier_idx[k]])
    end
    --app.unk_4c8:resize(33)--33
    app.tissue_style:resize(#caste.bp_appearance.style_part_idx)
    app.tissue_style_civ_id:resize(#caste.bp_appearance.style_part_idx)
    app.tissue_style_id:resize(#caste.bp_appearance.style_part_idx)
    app.tissue_style_type:resize(#caste.bp_appearance.style_part_idx)
    app.tissue_length:resize(#caste.bp_appearance.style_part_idx)
    app.genes.appearance:resize(#caste.body_appearance_modifiers+#caste.bp_appearance.modifiers) --3
    app.genes.colors:resize(#caste.color_modifiers*2) --???
    app.colors:resize(#caste.color_modifiers)--3
 



    unit.id=df.global.unit_next_id
    df.global.unit_next_id=df.global.unit_next_id+1
    df.global.world.units.all:insert("#",unit)
    df.global.world.units.active:insert("#",unit)
end


create_creature(args[1], tonumber(args[2]) or 0)