Mostly copied from the Unused Type Combos thread but with more ideas then my post there:
Ground/Fairy: A living labyrinth snake made of stone that lives in the desert. It tricks people into entering it. If they earn its respect, it'll let them out. This is something I REALLY want.
Electric/Dark: Either a cheetah or a lynx.
Electric/Poison: An anemone. Just because it's a sea creature doesn't mean that it HAS to be Water type.
Normal/Ghost: A living carpet or bed. It sounds silly now, but...
Normal/Rock: Sandworm based off the fantasy type that swim through sand.
Poison/Psychic: Earwig. They look creepy. Or... A brown bat.
Psychic/Bug: Mayfly, Blattodea (it is a black bug with white spots, kinda looks like a big lady bug), green lacewing.
Ice/Bug: A beautiful, crystalline bug that resembles a snowflake.
Bug/Dragon: Moth or butterfly dragon. Preferably a moth dragon.
Bug/Dark: Either a dung beetle, garden beetle or bed bug.
Poison/Steel: A battery or a machine-like Pokemon.
Rock/Ghost: A living marble pillar from an ancient age.
Here's some other ideas for type combos and mono types already used:
Dark/Poison: Centipede, Wasp, Hornet or Jerusalem Cricket.
Ground/Bug: Antlion or mole cricket.
Ghost: A music box! Depending on its mood, it'll play happy tunes, sad tunes etc.