Galthak are many things: armored, hard to kill, and pretty dumb. One thing they aren't is subtle.
These heavily-armored, insectoid creatures about the size of a small human scuttle through cave systems in search of food. Their biology remains unstudied, as their diet appears to be random. One day they'll eat fungus, and the next ignore it. One galthak might leap at an adventuring party, spitting acid and devouring its members in huge bites, while the next might behave in an entirely docile and even trainable manner (a fact circuses often exploit with "wild" galthak acts).
Thankfully, galthak are rarely encountered in groups. Even when they do prove dangerous, their behavior is sufficiently erratic they're not difficult to distract and at least run from.
I want the galthak to have a relatively high CR for a standard low-level adventuring party, so I'm going to base them on the CR 7 stone giant. I'll give it the same base stats, tweaking its skills and swapping the stone giant's rock attack for an acid spit. I'll also swap the stone giant's catch ability with a more flavorful spider climb.
Let's do this!
Homebrewery stat block:
Medium monstrosity, unaligned
- Armor Class 17 (natural armor)
- Hit Points 125 (16d8 + 61)
- Speed 40 ft.
STR DEX CON INT WIS CHA 23 (+6) 10 (+0) 20 (+5) 10 (+0) 8 (-1) 9 (-1)
- Skills Intimidation +3, Perception +4
- Senses passive Perception 11
- Languages --
- Challenge 7 (2,900 XP)
Spider Climb. The galthak can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Multiattack. The galthak makes two melee attacks, of which only one can be bite.
Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 16 (3d8 + 3) piercing damage.
Claws. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 6) slashing damage.
Acid Spit. Ranged Weapon Attack: +9 to hit, range 30/120 ft., one target. Hit: 16 (2d10 + 6) acid damage plus ongoing 2d10 acid damage (DC 17 Constitution save to end).