From the MM page 162, we know that Gnolls are created via hyenas that eat the flesh of a creature slain by a Fang of Yeenoghu. But the rituals that are used to create a Fang of Yeenoghu are not detailed, and left for a DM to create, if he feels it is necessary.

To that end, I am thinking it follows there has to be a Cleric of Yeenoghu, that performs the rituals. I am visualizing something like a 5th level Cleric/Shaman. What kind of spells would this Cleric have? I know it would have the Incite Rampage ability, as do many Gnoll leaders. I have to sort out some other Innate feature this monster would have.

Spells it would certainly have: Inflict Wounds, Animate Dead (create Witherlings), Bane, Cause Fear.

Any more go to spells for such a monster?

You could give it a Crown of Madness-like ability (Jaws of Hunger?) that forced the target to make powered-up bite attacks against nearby creatures.

I believe that, like with most boons gnolls receive, Fangs are directly created by Yeenoghu himself, similar to how Maw Demons or Shoosuva are given to war bands, or how Flinds are created. 

However, clerics to Yeenoghu certainly would exist. Mordenkainen's Tome of Foes has this for demonic boons given by Yeenoghu.

Ability Score Adjustment: Up to a +4 bonus to Strength and Dexterity, with an equal penalty to Intelligence and Charisma

Signature Spells: Tasha’s hideous laughter (1st level), crown of madness (2nd level), fear (3rd level)

Yeenoghu’s followers form packs of cannibalistic marauders. They grow more like gnolls in temperament with each passing day. His most devoted followers gain the Gnashing Jaws action option and the Rampage trait, while cult leaders gain the Aura of Bloodthirst trait.

Gnashing Jaws. Melee Weapon Attack: bonus to hit equal to this creature’s proficiency bonus plus its Strength modifier, reach 5 ft., one target. Hit: 1d4 + this creature’s Strength modifier piercing damage.

Rampage. When this creature reduces a creature to 0 hit points with a melee attack on its turn, it can take a bonus action to move up to half its speed and make its Gnashing Jaws attack once.

Aura of Bloodthirst. If this creature isn’t incapacitated, any creature with the Rampage trait can make its Gnashing Jaws attack as a bonus action while within 10 feet of this creature.

Quote from TheHumbug >>

I believe that, like with most boons gnolls receive, Fangs are directly created by Yeenoghu himself, similar to how Maw Demons or Shoosuva are given to war bands, or how Flinds are created. 

However, clerics to Yeenoghu certainly would exist. Mordenkainen's Tome of Foes has this for demonic boons given by Yeenoghu.

Ability Score Adjustment: Up to a +4 bonus to Strength and Dexterity, with an equal penalty to Intelligence and Charisma

Signature Spells: Tasha’s hideous laughter (1st level), crown of madness (2nd level), fear (3rd level)

Yeenoghu’s followers form packs of cannibalistic marauders. They grow more like gnolls in temperament with each passing day. His most devoted followers gain the Gnashing Jaws action option and the Rampage trait, while cult leaders gain the Aura of Bloodthirst trait.

Gnashing Jaws. Melee Weapon Attack: bonus to hit equal to this creature’s proficiency bonus plus its Strength modifier, reach 5 ft., one target. Hit: 1d4 + this creature’s Strength modifier piercing damage.

Rampage. When this creature reduces a creature to 0 hit points with a melee attack on its turn, it can take a bonus action to move up to half its speed and make its Gnashing Jaws attack once.

Aura of Bloodthirst. If this creature isn’t incapacitated, any creature with the Rampage trait can make its Gnashing Jaws attack as a bonus action while within 10 feet of this creature.

 Thanks. I will look at that.