A while ago I made a list of all Moon/Sun related Pokemon:
Clefable: It's evolutionary line is stated to be from the moon and he fairy typing added in 6th gen was enough to catapult it to OU staple.
Ninetails: It's hidden ability is Drought, so it is possible as it relates to the sun, but probably the least likely on this list.
Noctowl: This evolutionary line is only found in the wild at night, cause you know...they're owls. So it wouldn't be surprising if it did get a mega in Pokemon Moon
Bellosom: It evolves with a Sun Stone and many of it's Pokedex entries mention it as well, including it's Gold and Heart Gold entries which state that it's dance is actually a ritual to summon the Sun.
Sunflora: It is a sunflower and all of the pokedex entries mention the sun
Espeon/Umbreon: The Sun Pokemon and Moonlight Pokemon respectively, their method of evolution was used to introduce the new mechanic in gen 2 of day/night cycles.
Lugia/Ho-Oh: Lugia is the guardian of the sea and is said to control the ocean currents, similar to how the moon influences them. Ho-Oh, while more of a stretch, is the Rainbow Pokemon and rainbows are caused by sunlight refracting off of water in the air. Sorta have to include Ho-Oh with Lugia though as they are a pair.
Solrock/Lunatone: The obvious pick, they are literally the sun and moon
Deoxys: Another extraterrestrial pokemon
Cherrim: It changes forms in sunlight with it's ability and all of it's pokedex entries also mention it.
Cressealia: The Lunar Pokemon
Darkrai: It is active during the new moon.
Beheeyem: Another evolutionary line of aliens like Clefable, so it wouldn't be out of nowhere if it got a mega.
Volcarona: Another pokemon referred to as the Sun Pokemon, it's fire literally replaced the sun after a large volcanic eruption spewed ash into the air. Fittingly, in Blaze Black/Volt White, hacks of BW, it's ability is Drought.
Heliolisk: It evolves with the sun stone, it's name comes from the greek word for sun, and it gets the ability Solar Power.