Thomas was repeating what was already jewish tradition. You can follow the scroiptures from the seraphim article and you will notice there are flying snakes throughtout the bible.
Theres a thought from Maimonedes the most famous jewish intellect on their culture and he confirms this tradition.
This conclusion is based on conflating the meanings of saraph (burning) and nachash (snake), on the basis that some snakes are described as 'saraph (burning) nachash (snake)'. saraph is used of 'snakes' that cause 'burning' (i.e. painful venomous bites), and seraphim is used of 'angels' that are 'as if on fire'. However, this is not a valid basis for concluding that nachash and seraphim are synonymous. The thing that 'saraph nachash' and 'seraphim' have in common is the 'burning' part, not the 'snake' part.
If I say the blue car, and elsewhere I talk about the blue ones, it doesn't mean that I'm still talking about cars just because something else is blue.