Sourcebooks such as Defenders of the Faith list deities for some of the creatures in the Monster Manual. In Faiths and Pantheons, the table on page 221 presents information on most of these deities using the format of the Forgotten Realms Campaign Setting.
    Not all of the deities from other sources exist in the Forgotten Realms. For example, Panzuriel, a deity worshipped by krakens, sahuagin, and other evil aquatic creatures, has no influence on Toril, and spells are granted in his name by Umberlee (and most of these creatures actually worship her directly rather than through this alias). Any deity not listed on page 221 of Faiths and Pantheons or in the Deities chapter of the Forgotten Realms Campaign Setting does not exist on Toril. Table FP-90: Replacement Monster Deities (below) is a listing of equivalent for these "absent" deities presented in Defenders of the Faith and other sources.

Table FP-90: Replacement Monster Deities

Deity	Replacement	Worshiped By
Blibdoolpoolp*	Umberlee	kuo-toa
Damh	Tapann	satyrs
Eadro**	Silvanus	locathah, merfolk
Emmantiensien	Silvanus	treants
Falazure	none	dragons
Hruba	Malar	chimeras
Io	none	dragons
Merrshaulk	Set	yuan-ti
Panzuriel	Umberlee	kraken
Ramenos	Laogzed	bullywugs
Remnis	Aerdrie Faenya	giant eagles
Rhiannon	Silvanus	sprites
Semuanya	Silvanus	lizardfolk
Sixin	Loviatar	xill
Syranita	Aerdrie Faenya	aarakocra
Toknana	Beshaba	leucrottas
Verenestra	Shiallia	dryads, nymphs
Yeenoghu***	Malar	gnolls

    * That's right, I have Blibdoolpoolp on this table, even though she appears in Faiths & Pantheons in the table on page 221. That's because she wasn't on the version of that table that I turned over; she was added later, and I don't know why. FR already has a CE deity of evil water creatures (Umberlee), so I'm not sure why we need another one (and yet Panzuriel, another evil monster water deity, was left on this table and not added to page 211, strange).
    ** Clerics of "Eadro" whose native environment is underwater have spear as their favored weapon instead of the maul.
    *** Yeenoghu is a demon, and some gnolls may worship him as a demon as described on page 6 of Faiths & Pantheons, so he ought to have an entry on page 221 of F&P. See my corrections to the deity table file for the stats.

An entry of "none" means that there is no one deity that grants spells in that deity's name. Creatures that would worship this deity choose an appropriate patron from those listed here or in the Forgotten Realms Campaign Setting. For example, shadow dragons or undead dragons that would normally worship Falazure (draconic deity of energy draining, undeath, decay, and exhaustion) would choose a deity like Jergal, Shar, or Velsharoon.