Flinx Fur

Snow Flinxes are pushovers and every one of them drops some, which makes the ice biome the first stop for anyone starting a summoner. The fur becomes the Flinx Fur Coat and the Flinx Staff, and those two together hand you a working minion setup before you have fought a single boss. Nothing else available that early gives a summoner both a weapon and a slot to put a minion in.

After the summoner gear it is material for the Deer Thing if you want to call Deerclops on your own schedule, and for Flinx Fur Blocks and the furniture set if you are building anything snow themed. Those three uses add up faster than you expect, and Flinxes are common enough that going back for more is never a trip worth dreading.