There are some situations where a PC's intelligence may drop to animal levels due to ability damage/drain/burn. I would like to know how they act and do they still have access to class features and feats that they have acquired? I would assume at this point they wouldn't be able to speak either since humanoid intelligence ends at 3.
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The closest I can find to relevant RAW is in the rules for monster races:
But this doesn't necessarily mean that a PC with temporary int damage should be made an NPC.
I'd definitely allow any feats they haven't lost the prerequisites for. (Animals have feats, after all.) Same with class features; only those explicitly requiring intelligence (such as wizard spellcasting) should be lost.
Can they use skills? Can they speak, and understand speech? Well, in 3.5 you don't retroactively lose or gain skill points due to a change in intelligence. So, I guess strictly speaking yes! (And again, animals get skill ranks.) The PC will probably have more ranks than an animal, but this isn't as strange as it seems -- intelligence is described as "how well your character learns and reasons." Losing intelligence doesn't necessarily make you forget what you've already learned.
I'd probably use the pathfinder rules (which were certainly a common houserule in 3.5), and say that permanent changes do have an effect on skills and languages, Flowers for Algernon style.
So, other than simple penalties on Int based checks, and the loss of any abilities with an Int prereq, there aren't many mechanical effects here. It should absolutely have a large effect on how the PC acts (really large! they're dumber than a toddler!) but that's more of a roleplaying thing. I suspect most players would play it for comedy, but tragedy is always an option.
I would hand them a cookie and pet them on the head because they have now become the party pet.
Seriously though, you are right to an extent. It would be best to treat them as a toddler. With Intelligence, that is for the most part "book learnin" they can still function on many levels but they would lack the ability to use many things that require actual teaching. They would likely lose access to many (if not all) class features. I pity the Wizard who suffers enough Int damage to get that low.
They would retain many abilities that require little or no thought behind, muscle memory might factor in as well in the case of Fighter abilities and such. Anything with an Int requirement is most certainly a No-Go. They can still use their other ability scores, and so many things that depend on them wouldn't be affected unless the lack of coherent language and study would affect it. Skills would pretty much be out though, as would anything depending on said skills.
Barbarians are fine, most likely Fighters would be fine, but anything that requires skill and training beyond muscle training would have to suffer.
RP aspect wise... Yeah... Have fun with that.
Full disclosure: I started with the TSR Red Box set, and played heavily through 2nd Ed, as well as many other systems. I've looked at but not played 3.5, and haven't looked at 4 or Pathfinder, so I acknowledge that the mechanics may have changed drastically in the intervening years.
Also, all of this presumes some sort of temporary incapacity (magic, psi, poison) as opposed to the result of actual brain damage, and is predicated upon your statement of "animal-level", and the implication that you're talking about an INT score below 3.
Just because a skill or talent isn't explicitly "Intelligence-based" doesn't mean that the character will be able to still use it. Think about the impact that suddenly losing the ability to actually think, as opposed to simply react instinctively, would have on someone that has spent their entire (conscious) life thinking. What you're describing is not just "oh, wow, I don't feel so good". You're describing being knocked below the level that defines sentience.
A big critter that has low (i.e. animal-level) INT but high STR can certainly do lots of damage, and fight effectively, but its fighting abilities are based on instinct & natural abilities (and, admittedly, some learning, within the limits of that intelligence), not what we would class as "tactics" or "strategy". The animal is usually fighting with natural weapons instead of using tools.
A warrior's fighting is based as much on mental abilities as physical. Certainly, you must be able to swing the sword hard enough, but you have to know where and when to swing as well. Yes, INT is defined as a character's ability to learn, but it also encompasses memory, thought processes (instinct v. deliberate action or reaction, i.e. tool use), etc.
A great swordsman that suddenly lost his ability to process input wouldn't necessarily remember what that big metal stick strapped to his side was even for; to be able to take direction from his friends ("Charly, use your sharp stick to hurt the bad guy!") and fight effectively (v. covering his eyes and screaming in fear because he doesn't understand what's going on) just wouldn't be reasonable. At the minimum, I would be inclined to require an INT check each round (action, whatever they call it now) as the fighter's "natural" intellect tries to battle past the incapacity.