Only species with CAN_BE_FAT can, well, be fat.

code/modules/client/preferences.dm

@@ -16,7 +16,7 @@ var/global/list/special_roles = list( //keep synced with the defines BE_* in set
 	"infested monkey" = IS_MODE_COMPILED("monkey"),      // 9
 	"ninja" = "true",									 // 10
 	"vox raider" = IS_MODE_COMPILED("heist"),			 // 11
-	"diona" = 1,                                         // 12
+	"diona" = 0,                                         // 12
 )
 
 var/const/MAX_SAVE_SLOTS = 10
@@ -520,6 +520,8 @@ datum/preferences
 		return
 
 	proc/ShowDisabilityState(mob/user,flag,label)
+		if(flag==DISABILITY_FLAG_FAT && species!="Human")
+			return "<li><i>[species] cannot be fat.</i></li>"
 		return "<li><b>[label]:</b> <a href=\"?_src_=prefs;task=input;preference=disabilities;disability=[flag]\">[disabilities & flag ? "Yes" : "No"]</a></li>"
 
 	proc/SetDisabilities(mob/user)
@@ -761,8 +763,10 @@ datum/preferences
 					disabilities=0
 					SetDisabilities(user)
 				if("input")
-					if(text2num(href_list["disability"]) >= 0)
-						disabilities ^= text2num(href_list["disability"]) //MAGIC
+					var/dflag=text2num(href_list["disability"])
+					if(dflag >= 0)
+						if(dflag==DISABILITY_FLAG_FAT && species=="Human")
+							disabilities ^= text2num(href_list["disability"]) //MAGIC
 					SetDisabilities(user)
 				else
 					SetDisabilities(user)
@@ -1251,7 +1255,7 @@ datum/preferences
 			else if(status == "cyborg")
 				O.status |= ORGAN_ROBOT
 
-		if(disabilities & DISABILITY_FLAG_FAT)
+		if(disabilities & DISABILITY_FLAG_FAT && species=="Human")//character.species.flags & CAN_BE_FAT)
 			character.mutations += FAT
 		if(disabilities & DISABILITY_FLAG_NEARSIGHTED)
 			character.disabilities|=NEARSIGHTED

code/modules/mob/living/carbon/human/life.dm

@@ -869,22 +869,23 @@
 				heal_overall_damage(1,1)
 
 		//The fucking FAT mutation is the greatest shit ever. It makes everyone so hot and bothered.
-		if(FAT in mutations)
-			if(overeatduration < 100)
-				src << "\blue You feel fit again!"
-				mutations.Remove(FAT)
-				update_mutantrace(0)
-				update_mutations(0)
-				update_inv_w_uniform(0)
-				update_inv_wear_suit()
-		else
-			if(overeatduration > 500)
-				src << "\red You suddenly feel blubbery!"
-				mutations.Add(FAT)
-				update_mutantrace(0)
-				update_mutations(0)
-				update_inv_w_uniform(0)
-				update_inv_wear_suit()
+		if(species.flags & CAN_BE_FAT)
+			if(FAT in mutations)
+				if(overeatduration < 100)
+					src << "\blue You feel fit again!"
+					mutations.Remove(FAT)
+					update_mutantrace(0)
+					update_mutations(0)
+					update_inv_w_uniform(0)
+					update_inv_wear_suit()
+			else
+				if(overeatduration > 500)
+					src << "\red You suddenly feel blubbery!"
+					mutations.Add(FAT)
+					update_mutantrace(0)
+					update_mutations(0)
+					update_inv_w_uniform(0)
+					update_inv_wear_suit()
 
 		// nutrition decrease
 		if (nutrition > 0 && stat != 2)

code/modules/mob/new_player/preferences_setup.dm

@@ -145,7 +145,7 @@ datum/preferences
 			icobase = 'icons/mob/human_races/r_human.dmi'
 
 		var/fat=""
-		if(disabilities&DISABILITY_FLAG_FAT)
+		if(disabilities&DISABILITY_FLAG_FAT && current_species.flags & CAN_BE_FAT)
 			fat="_fat"
 		preview_icon = new /icon(icobase, "torso_[g][fat]")
 		preview_icon.Blend(new /icon(icobase, "groin_[g]"), ICON_OVERLAY)

code/setup.dm

@@ -308,9 +308,9 @@ var/MAX_EXPLOSION_RANGE = 14
 
 // Used in preferences.
 #define DISABILITY_FLAG_NEARSIGHTED 1
-#define DISABILITY_FLAG_FAT 2
-#define DISABILITY_FLAG_EPILEPTIC 4
-#define DISABILITY_FLAG_DEAF 4
+#define DISABILITY_FLAG_FAT         2
+#define DISABILITY_FLAG_EPILEPTIC   4
+#define DISABILITY_FLAG_DEAF        8
 
 	// Generic mutations:
 #define	TK				1