Here's what I've come up with for my gargantuan mutant dragon eel (this is in Xen'drik, so mutants are traditional

).
I took a neothelid and regressed it a couple of CR by applying -2 to its abilities, losing 3 HD, and lowering its natural armor. I then exchanged its special abilities for others that were similar but a bit weaker. There are a few ideas taken from the monsters as encounters suggestion, some things from 4e, and others that are just house rules. I wrote a few things up in shorthand as well, hopefully they aren't too nonsensical!
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Mutant Dragon Eel, gargantuan aberration (aquatic), CR 13
Init +2;
Senses Listen +26, Spot +26; DV 60’, blindsense 500’
AC 25, touch 11, ff 25 (-4 size, -2 Dex, +14 natural, +7 deflection)
hp 253 (22d8+154);
Bloodied 126, creates Ink Cloud & loses 1 tentacle
Ink Cloud 60’ total darkness immediately when first bloodied
Fast healing 5 at start of turn, regrows severed tentacles in 1d6 days
Fort +14,
Ref +5,
Will +14; Willful (can re-save vs mind-affecting next round)
Speed swim 50’, Burst (can charge 5x speed, or run 10x with no AoO and Backwash behind)
Melee full-attack 1 tentacle trip +21 touch, trip check +25, rake and Hurl
and 3 tentacle rakes +21 (2d6+9/19-20) and Improved Grab
Hurl on successful trip, can throw 10’/size difference in direction of choice
Backwash 60’ cone, trip check +21 and Hurl, if within 15’ take 3d6+9 from tail Reflex DC 30 for half.
Improved Grab +37 or +17, if two tentacles grapple same foe make immediate swallow whole attempt.
Swallow Whole +37 if starts round with creature in tentacle for rake damage & swallowed. No light, 2d6 acid, Shark In Gullet, Swim DC 15 (succeed 1d3s and act normally, or fail 1d6s and Swim or drowning), interior AC 17, 25 damage with light slash or pierce = get vomited out immediately.
Sonic Lance once/two rounds, 240’ ranged touch +10, 6d6 sonic and Fort DC 25 or dazed one rnd.
Keening once/encounter, 120’ spread, Will DC 25 or stunned for duration +1d4 rnds. If victim attacked or violently shaken (full-round action) they save again. Mind-affecting sonic compulsion, 1/24 hrs
Attack options Power Attack, Cleave, Combat Expertise, Improved Trip, Dire Charge (pounce on surprise round and first round of combat), Swim-by Attack (1 move, standard anywhere)
Str 28, Dex 7, Con 25, Int 14, Wis 13, Cha 18
Space/reach 30’/30’
BAB/grapple +16/+37
Feats Power Attack, Cleave, Combat Expertise, Improved Trip, Improved Critical, Improved Initiative, Dire Charge, Swim-by Attack
Skills Listen +26, Spot +26, Hide +8, Swim +39 (take 10, can run)
Shark In Gullet as Dire Shark, squeezing and can’t see in darkness
AC 13, touch 6, ff 11 (-2 size, +2 Dex, +7 natural, -4 squeezing)
hp 114 of 147 (18d8+66);
Bloodied 73
takes 7 acid damage per round
Fort +14,
Ref +13,
Will +12
Melee bite +14 (2d8+9) and improved grab +27
Swallow Whole +27 if it starts round with a creature in its bite for bite damage and swallowed and still grappled, 2d6+6 crushing plus 1d8+4 acid per round, interior AC 13, 25 damage with light slash or pierce = get vomited out immediately.