I got an old one from my little sister, it doesn't say what the temperatures are but I use it on the lowest setting (1, with 6 being the highest), it feels hot enough for me.
(I have to be careful not to touch the hair immediately after or it burns my fingers... but I haven't had any problems with melting, even if the hair gets stuck for a few seconds.)
For comparison, Fimo clay is baked at 110 Celcius. DH nylon should take this temp easily, but they say Hasbro ponies don't necessarily take baking... On the other hand, you would usually leave the pony in for some time. But you can definitely boil hair, so I'd say up to 100 is definitely safe.