Answers

• Intelligent items can be randomly rolled. A DM adhering strictly to random treasure generation will find that nearly any permanent magic item can be at random an intelligent item. That is, the Dungeon Master's Guide says on page 216 that 1% of armor, 1% of shields, 5% of ranged weapons, and 15% of melee weapons that are randomly generated are intelligent items; likewise on page 229 it says 1% of rings, on page 234 1% of rods, and on page 246 1% of wondrous items that are randomly generated are intelligent items.

  While that may seem like a lot of intelligent items waiting to be looted (and, depending on one's point of view, enslaved), it ends up being surprising few: the Dungeon Master's Guide's rules for random treasure generation (see Table 3–5: Treasure on 52-3) are fairly tight-fisted magic-wise. Even a CR 20 creature like a pit fiend, for example, has a 25% chance of possessing no magic items at all in its hoard. Create a few sample hoards using the DMG's rules (I suggest a good rules-bound online generator—this one's pretty good, for instance—instead of actual dice; doing so by hand gets tedious otherwise), and you'll rarely roll up an intelligent magic item.

• The DMG has no special rules for creating intelligent items. Except for mandating a minimum caster level of 15 and that the ensuing intelligent item have its creator's alignment (288), if a craftsman adds the cost from Table 7–30: Item Intelligence, Wisdom, Charisma, and Capabilities (269) and maybe at least one power from the Intelligent Item Powers chart (ibid.) to a permanent magic item, the magic item's intelligent. However, keep in mind that the Dungeon Master's Guide on 268-72 focuses primarily on how the Dungeon Master should add intelligent weapons to the campaign instead of on how the PCs can. Without consulting the DMG—y'know, a book for Dungeon Masters—, PCs don't even really know the price of a +1 dagger. Although the metagame lets players know a lot of things, the PCs themselves need never learn in your campaign that crafting intelligent items is even a thing if you don't want them to.

  I say this not to rain on anyone's parade but because intelligent magic items with special powers can severely disrupt the action economy. Moreover—and, perhaps, more importantly—, a DM probably doesn't want PCs crafting intelligent magic items willy-nilly because, each time a PC does, the PC adds another NPC the DM must manage. It's a little like having the feat Leadership, except every magic sword, amulet, rod, and boot a magical craftsman creates can become like a little cohort.

• The DMG doesn't say where an intelligent item's sentience comes from. However, this rules gap is filled in the Forgotten Realms campaign setting by the 6th-level Sor/Wiz spell Nybor's psychic imprint [trans] (Magic Books of Faerun column "Nybor's Small Codex: Spells from a Former Zulkir"), which, metaphorically, takes photograph of a creature's personality and uploads it into a magic item. That spell or one like it is as good an answer as any.

• PCs can't create artifacts and can modify them only with the DM's permission. The Epic Level Handbook says that "[m]ajor artifacts… are beyond the means of even epic characters to create" (73) and "minor artifacts are… magic items that no longer can be made by common mortal means—even by the hands of epic creators" (151). Even in the magic-saturated Forgotten Realms campaign setting, the Ars Factum chapter of the Nether Scrolls which "reputedly… taught the reader how to create artifacts" remains locked with all unable to discover the key to its powers (Lost Empires of Faerûn 156-7).

  Nonetheless, a generous DM may allow a PC to upgrade an artifact to sentience in the same way that a creature can pay to have a magic item further ensorcelled with more powers. This seems unlikely and dangerous, as artifacts—especially major artifacts—are important, and messing around with their magics is liable to alert someone or, worse, irritate someone.

More information about intelligent magic items—like how they can gain class levels but also much more than that (honestly, I went a bit overboard)—is available in answer to this question.

No, there are no such rules for sunder in particular

Excepting special cases for particular monsters, as with hydras or krakens.

The rules for sunder explicitly begin with

Sunder

You can use a melee attack with a slashing or bludgeoning weapon to strike a weapon or shield that your opponent is holding.

^{(emphasis mine)}

There is an additional section for a carrier or worn object, but that still doesn’t apply to natural weapons.

In online discussions of sunder, the inability to use it on natural-weapon-using monsters is frequently brought up as a massive problem with the tactic (though far from the only one). I have never seen anyone mention any official rule for sundering them, nor have I ever seen such a rule myself, strongly supporting my broader claim that no such rule was published in a supplement. I specifically checked Rules Compendium, as the most likely location of such a thing, and it only reprints what core had to say (plus a little sidebar on how a DM should prepare encounters when a PC is using sunder).

As for houserule, I have not used, or seen used, any particular rules. Most seem willing to accept that sundering is just a dead rule, that is undesirable for PCs and (often) obnoxious and unfun for use by NPCs. There have been few, if any, attempts to rehabilitate it, and I’m not familiar with any. Your approach of using the hydra’s rules seems appropriate, though I wouldn’t guess that bone and hide, even fantastic bone and hide, has the same Hardness as steel.

There may be optional, variant rules for called shots, but....

I have to admit that I am not familiar with any official variant rules for called shots, but I suspect they exist (and homebrew versions certainly exist as well). However, I would caution against them: they are, in effect, like super-charged criticals. Many even run off of critical mechanics, but even when they don’t, by definition you are talking about something with a lower chance of a higher consequence.

The problem with this is that it degrades the stability of the system. The ability to predict consequences and prepare for them is diminished, and the game was already quite swingy. I would argue, then, that these effects are to the detriment of the game.

You may disagree; you might want something even swingier. That’s fine, as long as you have a group that’s on board with that, but I think it is important that you and your group all know the ramifications here: swinginess is inherently bad for the players. Mathematically, a dire consequence as a result is equally likely for PCs and NPCs (assuming they’re making similar numbers of attempts, which seems mostly reasonable), but while such bad luck against NPCs is much more likely than not to befall some random mook, the same bad luck against PCs will land on a PC 100% of the time by definition. And since there are relatively low risks of these consequences, it is more difficult to defend against them—player resources are already devoted to a number of things they need to defend against as it is, so it will be difficult to justify diverting some of them to protect against a low-risk event.

Called shots are far better than critical or fumble tables in that they are specific, planned maneuvers, so they avoid a lot of the huge narrative failings of critical or fumble tables. So that much is good. And since you are presumably giving up an attack or something else to attempt these called shots, your odds can be reasonable, and if the odds are reasonable, the effects need not be so dire. So a well-made system could mitigate some of the mathematical problems I mention above. But it is important that, whatever you choose, you go into it with eyes open, aware of potential problems. Unlike critical or fumble tables, they’re not insurmountable for a group that’s interested in this sort of thing, but they’re not something to add on a whim because it sounds cool. Consider it carefully.