I just ran an old Dungeon adventure last week that called for some Small earth elementals. I found it easy enough to take the Large size elemental in the MM and rejig it to be Small using the guidelines in the DMG. The main thing is just to reduce the hit die size to a d6. This is what I came up with:
SMALL EARTH ELEMENTAL
Small elemental, neutral
Armor Class 15 (natural armor)
Hit Points 27 (6d6 + 6)
Speed 20 ft., burrow 20 ft.
STR 17 DEX 8 CON 13 INT 4 WIS 10 CHA 5
Damage Vulnerabilities thunder
Damage Resistances bludgeoning, piercing, and slashing
from nonmagical weapon attacks
Damage Immunities poison
Condition Immunities exhaustion, paralyzed, petrified, poisoned, unconscious
Senses darkvision 60 ft., tremorsense 60 ft., passive Perception 10
Languages Terran
Challenge 1 (200 XP)
Earth Glide.
Siege Monster.
ACTIONS
Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning dmg.