When reading a monster stat-block I noticed some spell-like abilities list a percent after the description. Does this mean that only X% of these monsters have this ability? If not, can you please tell me what it means?
For example, the Mephits' stat-block has this:
Spell-Like Abilities (CL 6th) 1/day—summon (level 2, 1 mephit of the same type 25%)
The information you need is in The Universal Monster Rules.
Summon (Sp) A creature with the summon ability can summon other specific creatures of its kind much as though casting a summon monster spell, but it usually has only a limited chance of success (as specified in the creature's entry). Roll d%: On a failure, no creature answers the summons. Summoned creatures automatically return whence they came after 1 hour. A creature summoned in this way cannot use any spells or spell-like abilities that require material components costing more than 1 gp unless those components are supplied, nor can it use its own summon ability for 1 hour. An appropriate spell level is given for each summoning ability for purposes of Will saves, caster level checks, and concentration checks. No experience points are awarded for defeating summoned monsters.
So, it is a chance of success of each separate use of this ability. You roll d% each time a creature uses the ability to determine if it was a success or not.
