I'm drawing my information for this answer from my reading of the doctoral dissertation of Amira El-Zein, titled "The Evolution of the Concept of the Jinn from Pre-Islam to Islam". I have attempted to summarize extremely briefly a few salient bits, particularly from pages 65-98. However, as it turns out, this topic is actually fairly complex, so giving it a read would be recommended: The Evolution of the Concept of the Jinn from Pre-Islam to Islam.

Jinn is a classification of a number of different types of supernatural being. They may be either good or bad, and there is no clear demarcation between types of jinn as either strictly good or strictly evil.

They could appear as pure spiritual beings, as stand ins for angels or demons, as indicated by Ibn Manẓūr, in "Lisān al-ʿArab"

The people of the Jahiliyyah called angels-peace be upon them-jinn because they were invisible from sight

They could also be tangible creatures, particularly with animal-like characteristics, as stated by "The Encyclopedia of Religion and Ethics"

...were not pure spirits, for they were often represented as hairy and often as having the form of an ostrich or a snake."

Four specific types of Jinn are detailed as significant in the pre-islamic world:

Ghūl - Quoting Al-Jāḥiẓ "Every supernatural creature that takes diverse forms". They seem to appear in many different forms, often animalistic, and can appear and disappear "like the light of a torch". They (and generally any jinn, it appears) leave a physical corpse behind when killed, thus the poem by Ta'abbata Sharran:

I spent the night leaning on her, waiting for the morning to see what I caught. Then I found two eyes in an ugly head, similar to the head of a cat with a forked tongue.

Si'lah - The difference between this and the former is not clear to me. They seem to be more sneaky and clever. A story exists of a one marrying and bearing human children. Possible other differences may be that they are active during the day, instead of at night, or that the Se'lah is female, and the Ghūl male. neither seems to be universally respected though.

Shiqq - A man with only half of a body, and will beat a human to death if encountered. Again, from Al-Jāḥiẓ:

There, a shiqq appeared to him. He had only one hand, one eye, one leg. They recited poetry to each other, then they hit each other, and both of them fell dead.