The following troops are currently not in chests:
Dokkalfar - Reward from 12th July 2015
Swamplash - Reward from 27th July 2015
Goblin Rocket - Reward from 10th August 2015
Elf Eater - Reward from 24th August 2015
Faunessa - Reward from 21st September 2015
Fenrir - Reward from 5th October 2015
Skeleros - Reward from 19th October 2015
Salamander - Reward from 2nd November 2015
Green Slime - Reward from 16th November 2015
Anointed One - Reward from 14th December 2015
All above troops will be added to glory/gem chests with the 1.0.8 Update (source)
On average troops are added to chests just under 5 months after being introduced as a reward.
Seasonal Imps should be available every year for 3 months each:
The current Imp is Winter Imp.
Spring Imp should appear March 2016,
Summer Imp should return to chest June 2016.
Autumn Imp should reappear in September 2016.
Hope that helps some people with troop availability confusion