Here is an old favorite of mine that I have done a couple of different versions of in various postings and books. Is this the final version? No idea! But it is getting close.
This one is a nasty little beastie from English lore.
Barghest
Large Beast (Demonic)
Frequency: Very Rare
Number Appearing: 1d6 (1 or 1d4)
Alignment: Chaotic [Chaotic Evil]
Movement: 180' (60') [18"]
Humanoid: 120' (40') [12"]
Hybrid: 150' (50') [15"]
Armor Class: 3 [16]
Hit Dice: 6d8+12** (39 hp)
Large 6d10+12** (45 hp)
THAC0: 13 (+6)
Attacks: 2 claws (humanoid/hybrid) or 1 bite (dog/hybrid)
Damage: 1d6 x2 claw or 2d4 bite
Special: Stare, hit by silver or magical weapons.
Save: Monster 6
Morale: 10 (10)
Treasure Hoard Class: None
XP: 950 (OSE) 980 (LL)
A Barghest is an evil shape-changing fiend that hungers for the souls of mortals. A barghest may appear as a huge demonic black dog the size of a bear, or in a humanoid form nearly seven feet tall, resembling a goblin or wingless gargoyle, or a combination of both forms. A barghest never uses weapons, even in its humanoid form, preferring to feel the blood of its enemies run down its claws. It is tenacious; if a barghest fails its morale check and flees, it will return in 1d6 turns to attack again.
Anyone who meets the gaze of a barghest will feel the heat of the monster's stare; such characters must save vs. Paralysis or be paralyzed in terror for 1d6+1 turns (or until the barghest is slain). A character is deemed to have met the gaze of the barghest if he or she faces it in combat, or if the character is surprised by the monster. Fighting a barghest with gaze averted results in a penalty of -4 on all attack rolls. Those who succeed at the saving throw are immune to the monster's gaze for the remainder of the combat (at least one full turn at the minimum).
Although it is not undead, a barghest is inherently unholy and can be Turned by Clerics (as a spectre). They can only be harmed by silver or magical weapons. A barghest generally speaks Common as well as the languages of infernals, goblins, hobgoblins, and bugbears, and can communicate with wolves. One can sometimes be found ruling over goblins or hobgoblins, but most commonly a barghest haunts a lonely stretch of road, preying on travelers.
Barghest lairs will only have a single creature or a creature and up to three whelps. A barghest whelm is weaned at one year and kicked out of the lair. If encountered by a parent or siblings it will be attacked. Barghests have no sense of family and hate all creatures except for themselves.
Their coats are of the darkest of blacks and often matted with blood. Their eyes burn red and it is said the fires of hell can be seen in them.
--
What's new today?
This creature has a few more things going on.
First, it is a Large Beast (Demonic). Let's break that down.
In my Basic Bestiary, I am going to give different HD for different sized creatures. A Large creature will use a d10. Medium creatures will still use a standard d8, Small a d6, and Tiny a d4. On the other side of things, a Huge creature will use a d12 and a Gargantuan creature will use a d20. This is much the same as D&D 5e uses.
For the purists, you can continue to use the d8 but I will include both numbers as I am doing above. AD&D First Edition only used Small, Medium, and Large creatures. The vast majority will be these three sizes.
It is a beast, but also Demonic. So it's intelligence is higher and has some demonic traits. In this case, it can change shape, can only be hit by magic or silver, and has a special gaze attack.
I am also including a THAC0 line with a BAB in parentheses.
Things are shaping up.