presume any appearance that doesn't confer a game effect is available.
Shovel-like hands that don't make the eidolon dig any faster, sure
cavity for your legs that do not provide any cover or protection bonus is fine too.
If the weirdness has a game impact, it must be purchased with a feat or evolution point, short of that, go wild.
I don't really see a problem with that, so long as your eidolon has the mount and climb evolutions and you're willing to be treated as any other halfling/eidolon mount+climb combination. The prone condition, however, only applies when you are on the ground, per the description for the condition:
The character is lying on the ground. A prone attacker has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A prone defender gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.
Standing up is a move-equivalent action that provokes an attack of opportunity.
So generally you can't have the prone condition atop a mount even if you're prone atop a mount.
Edit: I'd save the shovel hands for whenever you take the burrow evolution. The mouths thing is fine, but for mechanical purposes it's basically only one unless you take the head evolution. So it'd only get one bite attack, it couldn't activate two hypothetical command-word-free-action items simultaneously, etc.
Yes and no. You can put all of the individual attributes in, but you still end up making a creature that is obviously not the source creature, per the rules for eidolon apperance:
The eidolon’s physical appearance is up to the summoner, but it always appears as some sort of fantastical creature. This control is not fine enough to make the eidolon appear like a specific creature. The eidolon also bears a glowing rune that is identical to a rune that appears on the summoner’s forehead as long as the eidolon is summoned. While this rune can be hidden through mundane means, it cannot be concealed through magic that changes appearance, such as alter self or polymorph (although invisibility does conceal it as long as the spell lasts).
For example, you can make your eidolon have all the properties of a horse--mane, hooves, and so on--but anybody who has seen a horse will know that the eidolon is not a horse without even getting into the summoner's rune.
You can absolutely go nuts with creativity. The group I'm playing in has a summoner whose eidolon is a zebra-striped Shetland pony with clawed feet.
It is the "always looks like some fantastical creature" bit. So you can't have a horse, but maybe a pink unicorn.
Plus, we have generally agreed that you can't alter the appearance to give it mechanical benefits. Making it look exactly like a giant spider (the actual, in game creature) would confuse the knowledge checks to identify if you covered up the rune. You an make it look 'like' a giant spider, and heck, some know nothing commoner without any ranks in the appropriate knowledges couldn't tell the difference (although, with the horse example, I would give the commoner a DC 5 for the knowledge check since it is a common animal). But actually making it look like a giant spider well enough to confuse a knowledge check could require the disguise skill.
Admittedly, you could just get the skilled (disguise) evolution and just give your creature minor shapeshifting (look up the mimic octopus for a real world example of this method) to cover looking like the creature you want it to. I would still say that there are limits to even that, but they are mostly about which evolutions you have compared to the target's shape (looking like some type of bird while not having wings would be rather hard)
but it always appears as some sort of fantastical creature
You could posit that so long as the thing you're mimicking is already a fantastical creature, you could mimick that creature type perfectly, but I'd have to conclude that the intent was that someone who had seen a giant spider would be able to tell the difference just as someone who had seen a horse would. But that gets into RAI, so you may not see it that way. Fooling stablehands into thinking an eidolon is a horse is pretty much out, though, barring magical effect or a really good Disguise roll.
When in doubt, add more eyes, limbs, and horns.
When I said shapeshifting, I meant for the disguise skill's handwave so you can look like something "normal". With that, the no nothing commoner would have to do really well on a perception check to notice that the horse he found has tentacles slipping out occasionally or that its head is not quite the right shape and its legs too long
When in doubt, add more eyes, limbs, and horns.
Also works to houserule that fantastic creature part out. Bit arbitrary anyway.
Essentially, with the right evolutions, you can fool those dang nosy kids that were passing tales about the six legged bearfish around the campfire, but not someone who has throughly read and written a desertion about Ichthus-Ursidae Sexpoddes, even if he has never seen a live one in person.
For the horse analogy, it is like you are using a donkey. Sure, they might be superficially similar, but the details seem off.
but it always appears as some sort of fantastical creatureYou could posit that so long as the thing you're mimicking is already a fantastical creature, you could mimick that creature type perfectly, but I'd have to conclude that the intent was that someone who had seen a giant spider would be able to tell the difference just as someone who had seen a horse would. But that gets into RAI, so you may not see it that way. Fooling stablehands into thinking an eidolon is a horse is pretty much out, though, barring magical effect or a really good Disguise roll.The intent is that an eidolon is going to be obviously different from any creature you try to mimic. You might fool someone to think that it's a creature, but not to anyone who's seen the real thing.
You can not ride inside it. The only option for that is the synthesis archetype. You can ride ON the mount IF you have the mount evolution purchased for it.
The cosmetic change becomes a violation the moment it impacts on mechanics, whether by alteration, negation, or substitution.
Exactly. As a player, you can flavor it however you like (though you may have to keep it in your head if its PFS, as they don't really go for reskinning things), but as a character the summoner is subject to all of the mechanics of whatever features the summoner and the eidolon actually have. Even if you pretend the summoner is riding inside the eidolon (and hey, this game is all about pretending), enemies can still attack the summoner as if the summoner was astride a horse, and the summoner can fall off of the "saddle" under the same conditions as if you were riding it like a horse.