Becoming the Monster

[Fizban]

You've mentioned the flayerspawn feats (which have all kinds of editing problems), but the Aberrant Blood feats can also give you tentacles, as well as (bad) flight or swimming. You mention dragon breath and Dragonwrought Kobolds, but not Dragonborn from the same book, which is the only real source of non-class breath weapon. Or there's two classes with breath weapons.

Question too vague. What is "monstrous?" Why not just play a monster? Why not Totemist? What about one of the many homebrews for various monster concepts?

The Races of the Dragon Web Enhancement has kobolds with claws and bite. Darfellan from Stormwrack are just about the only PC aimed LA 0 race (that's not loldragon) with a natural bite, and it's a hefty full d6 too. From Planar Handbook, Neraphim have the toad-person look (Slaad blood) and a unique and useful dex denial ability, or the Lesser Bauriar is a mini-centaur.

Acolyte of the Skin (CArc). . . Green Star Adept is widely regarded as one of the worst prestige classes in the whole of 3E.

Which is funny, 'cause I'd take GSA over Acolyte of the Skin any day. At least GSA does something permanent for its loss of casting, where AoS gets a couple bad SLAs. People hate on GSA because (aside from caster-only mindset) it removes Con, but in a game without inflated ability scores, the loss is not nearly as great and could even be a boon, if you dared make a Raistlin. Or you can just not take the 10th level. Or make a magic item that boosts hit points for constructs. Or apply a one-sentence fix to whichever parts you find most offensive.

Acolyte of the Skin? Doesn't make you a half-fiend, random smattering of daily abilities, doesn't even have full caster level. Even Blood Magus beats it after the update, and updated Blood Magus is still terrible. Where GSA has a couple specific problems, AoS lacks any fundamental structure, you can't fix it without building a new structure to put the name on.