The term 'summoner' describes a special power of monsters in ADOM. In addition to their normal attacks and skills, summoners have the ability to magically call upon assistance. Different summoners can summon different types and numbers of monsters. Some summoners are capable of summoning more summoners; thus, their numbers can quickly get out of hand.
Summoners will get exhausted by summoning and stop for a while, but they will recover and resume summoning. If you or a pet of yours fight them, they will find less time to continue summoning, but the only way to stop summoners for good is killing them.
As of version r48, in addition to an increase to the frequency/duration of the 'cool down' period between summons, each type of monster that summons has a limit of how many monsters it can summon in total (though the counter for this may reset after a long time).