Of all the monsters of the that lurk in the underdark, aberrations are amongst the most feared. Between viscous, blood thirsty neogi, predatory and horrid grell, gibbering mouthers, the aboleths and their ancient chuul servents, stealthy cloakers, xenophobic beholders and their dream spawn, and perhaps most evil are the mind flayers- squidlike beings that devour peoples brains and infect intelligent life with their young to spawn more of them. 

Those that know of these horrors despise them. Many peoples in underdark have been hurt by these monsters, and are willing to hunt them. The members of these hunting guilds are often made up of brave and foolhardy duregar eger to seek revenge on their ancestral tormentors, strong willed svirfneblin eager to good in a bleak world, those rescued from the influence aberrations as well as goblinoids and dragonborn willing to profit of the selling of exotic monster parts. These guilds may be sponsored by the rulers of an underdark city, or a sapphire dragon as both have a vested interest in reducing the number of aberrations in an area.

These guilds use neothelids as mounts for several reasons. Firstly, thematically, neothelids are the last living remnants of an illithid colony, a sign of their failure and weakness. A second is practicality, members of this guild often train to be resistant to psychic powers and invisible to intelligence sensing creatures. Thirdly, they are a convenient method of expedient mass transport. The only problem is the supply of neothelids. Mind flayer colonies aren't exactly the most common things, and they fail only infrequently, and neothelids that do form are often hunted by mind flayers as they perceive them as threats, meaning that tracking down enough neothelids to service as mounts to guilds is difficult. 

The solution to this is to, after breaking a mind flayer colony guild members scour the elder brain's brine pool for a specific mind flayer tadpole, one that'd become a ulitherid were it implanted in a humanoid. When these tadpoles grow into neothelids, not only are they more powerful than other neothelids, but they are also capable of forming primitive brine pools and can produce illithid tadpoles the guild uses to produce sufficient neothelids for guild. They don't typically use these more powerful neothelids as mounts as their too valuable to risk losing and their higher intelligence leads them to being vulnerable to detection by elder brains.  

Aberration hunter guilds feed their mounts psychic grey oozes which they also capture and farm. This makes for a convenient food source and improves the neothelids psychic defences and deepens its connection to the far realm, letting it understand deep speech, greatly easing training, and makes it better at eating, violent squirming prey.

Aberration Hunter Statblocks. Listed below are examples of creatures that might be found mounting the neothelid. Feel fee to modify the number of these creatures and their stats to your needs. 

Duergar Worm Driver (Medium Humanoid (Dwarf))

Armour Class: 14 (scalemail)

Hit Points: 90

Speed: 30 ft

STR: 14 (+2)  DEX:12 (+1) CON: 16 (+3) INT: 14 (+2)  WIS: 16 (+3) CHA: 14 (+2)

Skill: Animal Handling +9

Saving Throws: WIS +5, INT +4

Damage Resistances: Poison, Psychic

Senses: Darkvision 120 ft., Passive Perception 16 

Languages: Deep Speech, Dwarvish, Undercommon 

Duergar Resilience. The duergar has advantage on saving throws against spells and the charmed, paralyzed or poisoned conditions. 

Spellcasting (Psionics). The duergar’s spellcasting ability is Wisdom (spell save DC 13). It can cast the following spells, requiring no material components:

3/day: calm emotions (can target Aberrations)

1/day: charm monster 

Actions:

Javelin. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 30/120 ft., one target. Hit: 5 (1d6 + 2) piercing damage, or 9 (2d6 + 2) piercing damage while enlarged plus 28 (8d6) necrotic damage against aberrations.

Invisibility. (Recharges after a Short or Long Rest). The duergar magically turns invisible until it attacks, casts a spell, or until its concentration is broken, up to 1 hour (as if concentrating on a spell). Any equipment the duergar wears or carries is invisible with it.

Enlarge. (Recharges after a Short or Long Rest). For 1 minute, the duergar magically increases in size, along with anything it is wearing or carrying. While enlarged, the duergar is Large, doubles its damage dice on Strength-based weapon attacks (included in the attacks), and makes Strength checks and Strength saving throws with advantage. If the duergar lacks the room to become Large, it attains the maximum size possible in the space available.

Aberrant Spellcaster (Medium or Small Humanoid)

Armour Class: 10 (15 with mage armor cast)

Hit Points: 70

Speed: 30 ft

STR: 10 (+0)  DEX:12 (+1) CON: 14 (+2) INT: 20 (+5)  WIS: 16 (+3) CHA: 10 (+0)

Skills: Arcana +7

Saving Throws: WIS +5, INT +7

Senses: Darkvision 60 ft., Passive Perception 16 

Damage Resistances: Psychic

Languages: Deep Speech, Undercommon 

Spellcasting. The spellcaster's spellcasting ability is Intelligence (spell save DC 19, +9 to spell attack rolls). It can cast the following spells:

At will: dancing lights, fire bolt, mage hand, nondetection

3/day: arms of hadar, intellect fortress, lesser restoration, protection from evil and good 

1/day: banishment, counterspell, dispel evil and good, earthen grasp, greater restoration, mage armor, planer binding

Actions: 

Quarterstaff. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 2) bludgeoning damage plus 28 (8d6) necrotic damage against aberrations.

 Svirfneblin Ranger (Small Humanoid (Gnome))

Armour Class: 15 (leather armor)

Hit Points: 80

Speed: 30 ft

STR: 10 (+0)  DEX:18 (+4) CON: 14 (+2) INT: 14 (+2)  WIS: 18 (+4) CHA: 10 (+0)

Skill: +6 Survival 

Saving Throws: WIS +6, INT +4

Senses: Darkvision 120 ft., Passive Perception 15

Damage Resistance: Psychic

Languages: Deep Speech, Gnomish, Terran, Undercommon 

Stone Camouflage. The gnome has advantage on Dexterity (Stealth) checks.

Gnome Cunning. The gnome has advantage on Intelligence, Wisdom, and Charisma saving throws.

Spellcasting. The gnome's spellcasting ability is Wisdom (spell save DC 16). It can cast the following spells:

At will: druidcraft, shillelagh 

3/day: cure wounds, detect magic, earth tremor

1/day: ensnaring strike, locate creature, pass without trace, spider climb, stone shape, web

Innate Spellcasting. The gnome’s innate spellcasting ability is Intelligence (spell save DC 14). It can innately cast the following spells, requiring no material components:

At will: nondetection (self only)

1/day each: blindness/deafness, blur, disguise self

Actions:

Club. Melee Weapon Attack: +2 to hit (+6 to hit with shillelagh), reach 5 ft., one target. Hit: 2 (1d4) bludgeoning damage, or 8 (1d8 + 4) bludgeoning damage with shillelagh plus 28 (8d6) necrotic damage against aberrations.

Short bow. Ranged Weapon Attack: +6 to hit, range 30/320 ft., one target. Hit: 7 (1d6+4) piercing damage plus 28 (8d6) necrotic damage against aberrations. 

Sapphire Dragonborn Pike-Master (Medium Humanoid (Dragonborn)

Armour Class: 18 (plate)

Hit Points: 90

Speed: 30 ft

STR: 18 (+4)  DEX:10 (+0) CON: 18 (+4) INT: 14 (+2)  WIS: 16 (+3) CHA: 10 (+0)

Saving Throws: WIS +5, INT +4

Senses: Darkvision 60 ft., Passive Perception 15

Damage Resistance: Psychic, Thunder

Languages: Deep Speech, Undercommon, Telepathy 30 ft

Actions: 

Multiattack. The dragonborn makes two pike attacks and one Thunder Breath.  

Pike. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 12 (2d6) piercing damage plus 28 (8d6) necrotic damage against aberrations. 

Thunder Breath (Recharge 5–6). The dragonborn exhales a shockwave in a 15-foot cone. Each creature in that area must make a DC 15 Dexterity saving throw, taking 10 (2d10) thunder damage on a failed save, or half as much damage on a successful one.

Bonus Actions.

Gem Flight. The dragonborn gains a fly speed equal to its walking speed for 1 minute.

 Goblin Mounted Mini-Ballista (Small Humanoid (Goblinoid)

Armour Class: 17 (half plate) 

Hit Points: 70

Speed: 30 ft

STR: 14 (+2)  DEX:18 (+4) CON: 16 (+3) INT: 14 (+2)  WIS: 16 (+3) CHA: 10 (+0)

Saving Throws: WIS +5, INT +4

Senses: Darkvision 60 ft., Passive Perception 15

Damage Resistance: Psychic

Languages: Deep Speech, Goblin, Undercommon

Nimble Escape. The goblin can take the Disengage or Hide action as a bonus action on each of its turns.

Enchanted Ballista. A mini-ballista is mounted to the neothelids saddle. Attacks made with this ballista are magical. If the ballista has been fired once this round the reload action must be used before it can be used again. 

Actions.  

Ballista. Ranged Weapon Attack: +10 to hit, range 100/400 ft., one target. Hit: 9 (1d10 + 4) piercing damage plus 28 (8d6) necrotic damage against aberrations. 

Reload. Allows the use of the Ballista action next round.

Remove Ballista. The ballista is removed from its mount on the neothelid's saddle, allowing it to be moved. While dismounted, it has the statistics of a heavy crossbow other than that it deals an additional 28 (8d6) necrotic damage against aberrations on a hit and its attacks are magical.