Giving simple_animal their own version of update_canmove(). This fixes small animals' density being always set to 1 by life().

Fixes pixel y shift when lying down as an alien.
Removes the no longer needed slime/update_icons.dm
Fixes alien icon var being reset to its initial value during every life(). I added an alt_icon var to aliens to achieve that.
Fixes alien humanoid no using the unconscious icon instead of their sleeping icon when sleeping.

code/modules/mob/living/carbon/alien/humanoid/humanoid.dm

@@ -4,6 +4,7 @@
 	var/obj/item/r_store = null
 	var/obj/item/l_store = null
 	var/caste = ""
+	var/alt_icon = 'icons/mob/alienleap.dmi' //used to switch between the two alien icon files.
 	var/leap_on_click = 0
 	var/pounce_cooldown = 0
 	var/pounce_cooldown_time = 30

code/modules/mob/living/carbon/alien/humanoid/update_icons.dm

@@ -19,18 +19,15 @@
 		//If we mostly took damage from fire
 		if(fireloss > 125)
 			icon_state = "alien[caste]_husked"
-			pixel_y = 0
 		else
 			icon_state = "alien[caste]_dead"
-			pixel_y = 0
 
-	else if(stat == UNCONSCIOUS || weakened)
+	else if((stat == UNCONSCIOUS && !sleeping) || weakened)
 		icon_state = "alien[caste]_unconscious"
-		pixel_y = 0
 	else if(leap_on_click)
 		icon_state = "alien[caste]_pounce"
 
-	else if(lying || resting)
+	else if(lying || resting || sleeping)
 		icon_state = "alien[caste]_sleep"
 	else if(m_intent == "run")
 		icon_state = "alien[caste]_running"
@@ -38,17 +35,21 @@
 		icon_state = "alien[caste]_s"
 
 	if(leaping)
-		icon = 'icons/mob/alienleap.dmi'
+		var/old_icon = icon
+		icon = alt_icon
+		alt_icon = old_icon
 		icon_state = "alien[caste]_leap"
 		pixel_x = -32
 		pixel_y = -32
 	else
-		icon = initial(icon)
+		if(alt_icon != initial(alt_icon))
+			var/old_icon = icon
+			icon = alt_icon
+			alt_icon = old_icon
 		pixel_x = initial(pixel_x)
 		pixel_y = initial(pixel_y)
 
 
-
 /mob/living/carbon/alien/humanoid/regenerate_icons()
 	..()
 	if (notransform)	return
@@ -63,8 +64,9 @@
 /mob/living/carbon/alien/humanoid/update_transform() //The old method of updating lying/standing was update_icons(). Aliens still expect that.
 	if(lying > 0)
 		lying = 90 //Anything else looks retarded
-	update_icons()
 	..()
+	update_icons()
+
 
 
 /mob/living/carbon/alien/humanoid/update_hud()

code/modules/mob/living/carbon/update_icons.dm

@@ -7,7 +7,6 @@
 	var/final_pixel_y = pixel_y
 	var/final_dir = dir
 	var/changed = 0
-
 	if(lying != lying_prev)
 		changed++
 		ntransform.TurnTo(lying_prev,lying)

code/modules/mob/living/simple_animal/simple_animal.dm

@@ -381,7 +381,6 @@
 	icon_state = icon_dead
 	stat = DEAD
 	density = 0
-	lying = 1
 	if(!gibbed)
 		visible_message("<span class='danger'>\the [src] stops moving...</span>")
 	..()
@@ -426,8 +425,7 @@
 /mob/living/simple_animal/revive()
 	health = maxHealth
 	icon_state = icon_living
-	lying = 0
-	density = 1
+	density = initial(density)
 	update_canmove()
 	..()
 
@@ -478,3 +476,12 @@
 		return
 	else
 		..()
+
+/mob/living/simple_animal/update_canmove()
+	if(paralysis || stunned || weakened || stat || resting || buckled)
+		drop_r_hand()
+		drop_l_hand()
+		canmove = 0
+	else
+		canmove = 1
+	return canmove

code/modules/mob/living/simple_animal/slime/update_icons.dm

@@ -1,4 +0,0 @@
-//no special icon processing
-
-/mob/living/simple_animal/slime/update_transform() //They're slimes, they don't lie down.
-	return

code/modules/mob/mob.dm

@@ -771,7 +771,7 @@ var/list/slot_equipment_priority = list( \
 
 
 //Updates canmove, lying and icons. Could perhaps do with a rename but I can't think of anything to describe it.
-//Robots and brains have their own version so don't worry about them
+//Robots, animals and brains have their own version so don't worry about them
 /mob/proc/update_canmove()
 	var/ko = weakened || paralysis || stat || (status_flags & FAKEDEATH)
 	var/buckle_lying = !(buckled && !buckled.buckle_lying)