The actual entities that you can summon are generally governed by the type of magic you are using and the cult/school that you belong to. Both spirit magic and sorcery have spells to facillitate this.
As a broad overview, summoning covers demons, spirits, ghosts and elementals, plus a couple of otherworldly entities that don't fall into those categories.
Cults vol 2 has stats for ancestral, animal, plant, intellect, power and magic spirits, plus The Bad Man, Parts of the Bull (Storm Bull Cult Spirits), Chaos Spirits of Thed and Thantari Guardian Spirits.
Other entities are in RuneQuest Monsters and the RuneQuest Companion.
Note that the actual rules for handling spirits and spirit combat are also in the Companion, not in the Cults book.