By 3e standards they are Abberations. In the same class as Beholders, Carrion Crawlers, Driders, Mimics, Mind flayers, nagas, and umber hulks among others.
The best way to think of abberations is as alien lifeforms from different planets or places with different rules. They're not insects and while they're not very smart they are 'intelligent' and can speak common. They're not mammals exactly but they share similar qualities. Skeleton surrounded by flesh is the biggest.
Also on the spider relationship bit, they keep the small spiders as pets much like we would keep bees and the large spiders like we would keep a dog.
So in answer, they are most similar to mammals without being a mammal.