For whatever reason they are chosen, the greater the variation in potential companions in a group, the more diverse the game - everyone with an owl familiar following Harry Potter, or all the mages in the group taking the animal with the greatest mechanical advantage is not what the game is about.
I'm talking about a cat sized, or smaller, arachnid, at least, at first. Bigger than this is arch-villain territory, and should not be encouraged, at least, not until higher levels. Smaller than this is likely to be squashed by travelling companions or animals.
- At lowish levels, there's the standard spider abilities - most notable being poison, should they bite anyone, and webs. At this stage, it's catching rats and brownies, and making tripwires only.
- Later on, at the mid-range levels, the poison is still there, but the web size allows it to trap small creatures (halflings and goblins, and slow dwarves), and create stronger and more effective tripwires and web-based traps.
- At high levels, the spider has probably grown a bit from it's rich diet of paladins and slain good-guys, and can make large and extensive webs, capable of trapping and biding warriors, and can even launch the odd net to entrap smallish flying creatures. I wouldn't recommend that it takes on a Griffin...
The issue here is sociability, as having a creature the size of a Great Dane following you around, sucking the liquefied innards of random stock, is off-putting to most potential comrades. Except for the general misfits that become successful adventurers... Nothing can shock them...
One of my favourite creatures, mainly because, as a child putting together an unholy melange of RPG systems based on what I had access to (MERP, Fighting Fantasy gamebooks, and bad 80s fiction), I was given a handful of roleplaying magazines one day - two Dragon magazines and a few White Dwarves. I had no context for most of their contents, but certain parts were lifted.
Between 6 inches and a foot long, it's diet consists entirely of slimes, moulds and mosses, and it is unharmed by any of their effects. It considers the more dangerous types more tasty, and does 2-8 damage a round to them as it hoovers them up through it's little proboscis. Also immune to the effects of Yellow Musk Creepers and Violet Fungi, even though it doesn't eat them. It's sleepingbag-like body inflates and swells as it feeds.
The Catalogue states that they are docile and faithful companions, provided they are given one full meal a day, and there is a trade in them amongst adventurers, for whom they will fetch up to 1,000 gold pieces. Their intelligence is "animal to low", so some of the brighter ones can manipulate tools clumsily with their forearms.
Anyway, as far as useful, exotic and thematic, there's definite potential for familiar status here. Probably to a mage that is a professional dungeon crawler, and is experienced in the underground realms.
- As a low-level companion, it will eat slimes, moulds and mosses that are encountered, but will suffer and sulk if not feed these often enough. Standard powers, in fact.
- In the mid-levels, it develops a taste for oozes and jellies as well, and gains extra immunities against other protoplasmic encounters.
- And for high levels, these immunities to slimy, mouldy, oozey things are conveyed to it's master, providing him or her a definite advantage when exploring ancient flooded tombs or robbing the vaults of past emperors.