OK, let's examine the three main combat functions of an actaeon. Some suggestions have been made about each, and I have examples on each from other conversions I have seen. These examples follow below; we can either take things from them, come up with the whole ability on our own, or both.
Mystara MC Appendix said:
The actaeon can camouflage itself perfectly (as if invisible) in light or dense woods. When angered by the wanton slaying of woodland creatures (or similar vile acts), it springs out of hiding, usually with surprise (opponents suffer a -5 penalty to their surprise rolls).
Skills: * All Actaeons receive an innate +10 Hide bonus when in light or dense woods, +4 innate bonus to Knowledge Nature and Wilderness Lore (Woodland Only).
Camouflage (Ex): It can camouflage itself perfectly (as if invisible) in light or dense woods.
Camouflage: If an actaeon successfully passes a Hide skill check, he is deemed as though he is under the effects of an invisibility spell if he is hiding in woodland areas. Attacking from hiding gives the actaeon an additional +5 bonus to his initiative roll. This ability is natural, so magic detecting spells will not pick up on the creature.
Skills: Actaeon have racial bonuses of +4 to Animal Empathy, Hide (bonus increases to +8 in woodlands), Move Silently (in woodlands only), and Wilderness Lore skill checks.
I vote for the ranger's camouflage ability, and give it a +4 racial bonus to Move Silently (in woods) and SUrvival and a +2 racial bonus to Handle Animal.
Camouflage (Ex): An actaeon can use the Hide skill in any sort of natural terrain, even if the terrain doesn’t grant cover or concealment.
Forest Stealth (Ex) : Within the forest, the acteon moves with uncanny agility and stealth. He recieves a circumstance bonus of +10 to hide checks, made within the forest.
Mystara MC Appendix said:
A powerful magical breath weapon complements the actaeon's other capabilities. Once per day, it can breathe out a warm, greenish mist, filling a 10 x 10 x 10 foot cube; anyone within it must make a saving throw vs. breath weapon or be polymorphed into a common forest creature, taking on the creature's intelligence and habits as well as its looks. ("Common" forest creatures include owls, squirrels, deer, boars, and the like) This change is permanent unless countered by another polymorph spell, or by dispel magic cast at 12th level or higher. If the saving throw is successful, the transformation still occurs, but it lasts for only 24 hours. The breath weapon can be used once per day.
Breath Weapon: Once per day, the Actaeon can breathe out a 10'x10' cone of warm greenish mist that forces any within the area of effect to make a Fortitude save DC 15, or be polymorphed into a common woodland animal of the DM's discretion. The effects are permanent until dispelled (Caster level 12), another polymorph spell, or wish.
Polymorph Breath (Su): It has a powerful breath weapon that can be used once per day, filling a 10 ft. by 10 ft. by 10 ft. cube; each victim within it must make a Reflex saving throw (DC 18) or be polymorphed into a normal forest creature (owl, squirrel, deer, etc.). If the saving throw is successful, the transformation still occurs, but lasts for only 24 hours.
Polymorphing Breath: One per day, the actaeon breathes out a green mist that cover a 10ft x 10ft x 10ft area. Anyone in that area must pass a Fort save (DC 20) or be turned into a common woodland creature (common as in owl, squirrel, deer, and so on). The effects are permanent unless subjected to another polymorph spell or a dispel magic spell cast by a 12th level or higher cleric, sorceror, or wizard. Even if the save passes, the victim still turns into a woodland creature, but the effect wear off after 24 hours.
Breath Weapon (Su): Once per day, the actaeon can breathe a cone of green mist. Anyone caught in the breath weapon must make a Fortitude saving throw (DC X) or be affected by a baleful polymorph spell at 11th level (forest creatures only). The save DC is Constitution based.
Breath Weapon (Su) : once per day, the acteon can breath a cloud of green vapour. The cloud fills a 10'x10'x10' area that is adjacent to the acteon. Any being within it is affected by a strong baleful polymorph. The save DC is 22, but even a successful save means a duration of 24 hours. The Acteon may breathe again the next day to dispel his own breath. Save DC is con based.
Mystara MC Appendix said:
Also once per day, the actaeon can summon woodland creatures to assist it; 1d6 creatures arrive in 1d4 turns. Choose from the list below or roll 1d6 to determine the creature type at random.
Actaeon Woodland Creature Summoning Table
Roll Creature
1 Boar
2 Bear
3 Centaur
4 griffon
5 lizard (chameleon)
6 treant
Spell-like abilities: Once per day, the actaeon may Call Animals as a 12th level Druid.
Summon Woodland Creatuers (Sp): Once per day an actaeon may summon woodland creatures to assist it; 1d6 creatures arrive in 10-40 rounds. Choose or randomly determine the type of creatures appearing:
1 boar
2 bear
3 centaur
4 griffon
5 lizard (chameleon)
6 treant
Summoning: Once per day, the actaeon can summon 1d6 woodland creatures to aid him. They arrive in 1d4 turns and are determined on the table below.
1 Boar
2 Bear
3 Centaur
4 Griffon
5 Lizard (chameleon)
6 Treant
Summon Nature's Allies (Sp) : Once per day, as a spell-like ability, the acteon may use a summon nature's ally VI effect.
