Batrachians and Beezlebubs

Two similar yet opposed classes for the old school rpg of your choice (though I sort of assume LotFP).

The Frog Demon
d8 Hit Dice (6 hp at level 1)
Attack Bonus: as cleric (I use 1/2 level, rounded up)
Saving Throws: as elf
Experience Progression: as elf

At level one, you are a fledgling frog demon who is really only a simple tadpole, able to manipulate objects with your mouth and able to swim and breathe in both air and water. In addition, you have a single cultist follower who you can order to do things for you. You gain 1 new cultist follower each level, and can gain more through normal hireling means and by converting people to your worship. If any of your cultist followers die, they are "refilled" back up to the minimum number you can have, which is equal to your level, the next day. You are able to speak with all frog creatures.

At level 2, in addition to the cultist follower that you gain, you grow legs and the ability to jump great distances.

At level 4, in addition to the cultist follower that you gain, you become a fully-grown frog demon. You are able to control normal frogs, and have a special tongue ability that allows you to grab a creature or object from a distance and pull it towards you. This functions as an opposed roll, with both you (the frog demon) and the victim rolling 1d20 and adding your attack bonuses. Both you and the victim add their STR bonus to the roll in addition to the attack bonus, and the one who rolls higher wins. Arthropod targets get a -2 to their roll.

At level 6, in addition to the cultist follower that you gain, you gain the ability to turn your cultist followers into frog monsters. You can do this once per day, increasing by 1 extra use for every level above 6. You simply point at the cultist which you are gracing with a better, slimier, amphibian form, and their body contorts and mutates into some sort of frog monster. The exact appearance is up to you, but the creature always has the same stats: 1+1 HD, +1 attack bonus, 14 AC, 1d8+1 damage. You are able to spend more than 1 "use" to increase the stats of the creature, increasing all stats by one increment (2+1 HD, +2 attack bonus, 15 AC, 1d10+1 damage, etc). All frog monsters use your saving throws rather than their own.

The Fly Demon
d6 Hit Dice
Attack Bonus: as thief (1/2 level, rounded up)
Saving Throws: as fighter
Experience: as elf

At level one, you are a little baby beezlebub, nothing more than a fat giant maggot. You can manipulate some objects with your mouth, and can crawl upon the ground. You can also use your tusk-like mouthparts to dig through certain materials, especially meat. You follow the same cultist follower rules as the frog demon above, and so begin play with 1 cultist.

At level 2, in addition to the cultist follower you gain, you begin to go inside of your pupa, and thus cannot interact with the world in any way. However, you float in the air and are able to communicate with the minds of your followers. You are considered as having an armor class of 9, but always take 2 fewer point of damage than normal, and your followers will probably protect you.

At level 4, you crawl out of your pupa as a fully-formed adult fly. You are able to fly in the air, control flies of all life stages (maggot, pupa, and adult), and gain an acid-vomit ability. Your acidic vomit deals 1d6 damage, and can deal more damage (or have some additional effect) if you have consumed some sort of special food recently. However, you lose the points of armor and low armor class from your pupal stage of development.

At level 6, in addition to the cultist follower you gain, you obtain the ability to turn your cultists into either fly monsters at some stage of a fly's life cycle, a swarm of flies or maggots, or a strange mutated fusion of human and fly. You can do this once per day, increasing by 1 additional use each level above 6. You point at one of your cultists, and their skin begins to split as a fly or fly mutant pulls itself out, or they burst into a swarm of flies or maggots. The exact appearance is up to you, but the statistics are always the same: 1 HD, +3 attack bonus, 14 AC, 1d6+1 damage. You are also able to spend additional "uses" to increase the stats (2 HD, +4 attack, 15 AC, 1d8+1 damage, etc.) All such fly creatures use your saving throws.