I wouldn’t have thought so - it’s only slightly low. also unless you take it regularly you won’t know what is normal for them. Maybe take it a few days consecutively to get an average temperature.
36.4 - 36.6 has consistently been the normal temperature for one of ours and he's healthy. As above, take the temperature at the same time each day for a week, and that will give an idea of their average / normal range.
I wouldnt worry, just take the temp every day for a week or so and get an idea of whats normal for the individual horse, one of mine has a very consistent normal temp of 37.3.