Inevitables are the exemplars of law. They are not outsiders, but are extraplanar constructs with an obsession to punish transgressors of law. The nature of the transgression dictates the type of the inevitable sent. Kolyaruts mete out punishment to those who break bargains and oaths.
Invisible stalkers are variant air elemental creatures that are naturally invisible and adept at tracking. To preserve the original flavor, rule that killing one on a non-native plane only banishes the invisible stalker to the Elemental Plane of Air instead of killing it.
Hydras are horrifying creatures - both as enemies for your characters to slay and as templates to stat up. Slaying one of these beasts requires a great deal of preparation and teamwork. Leaving it harmed but not killed will only make it stronger and harder to defeat in the future. Racial templates are given for 5-headed hydras, as this is the lowest head count given in Monster Manual.
Howlers are the bestial denizens of the Windswept Depths of Pandemonium, a plane scoured by mind-bending, deafening minds. They are animalistic, but intelligent.