There were a few things that would excuse summoning a demonic triceratops-looking dragon. The summoner would truly need to have their back against the wall in order to pull up such a desperate trick out of their sleeve.
It so happens, Fortana's back was put up against the wall. She was hiding in a hole, a couple of feet below the seafloor, and there were nearly two-hundred huge seahorses above her head who wanted to burn her alive! She blew out half of her mana pool capacity, but at least now she was certain that she and her friends would win the battle.
Summoning a Shanakas in this situation was an overkill. It was the most powerful beast that she could summon, and the fact of the matter was that these sea horses were far beneath the demon in terms of power scales.