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On a low temperature a clothes iron could act as a flat iron... as long as you don't mind hair product on your iron. Theoretically, anyways. Also, I'm pretty sure a g1 would melt at a lower temp than a g4. If anyone wants to send me some to test on, let me know. lolWhile I don't know the exact "melting point" of any pony, any exposure to a high temperature will effect a pony's vinyl integrity. A pony can melt or become deformed if left boiling too long (or if they touch the bottom of the pot). The sun can do all kinds of damage to a pony when left unattended.
When it comes to using heat with a pony, obviously, please always use extreme caution. High heat is usually only used in extreme cases when dealing with a pony and shouldn't be your first option. I'm not entirely sure why you would use a clothes iron on a pony, but a good resource for any pony restoration/cleaning is the MLP Preservation Project. This site has an accumulated a lot of knowledge from many collectors over the years and is a plethora of knowledge.