I did this as an experiment.

Large random map, commander sovereign, start game and beeline to the "reveal heroes" skill, quit and repeat.

I did this 10 times to see how many and what Secondary heroes would be in the game at frozen hero monuments.

results:  6 games with 2 additional heroes, 4 with 3 additional heroes. 

Hero occurrences:  Tan-9;   Nap-7  , Per-3  , Kep- 5, Dor-0

Could just be luck of the random draw but it doesn't look like normal distribution

Drogon was NEVER one of  them and i think that is a bug.  He may not have been added to the spawn.