Frog

The spawn rules are friendlier than they look. Lay jungle grass near your town and frogs will turn up on it even though there is no real jungle for a thousand tiles, and town spawn rates are generous to begin with. Do it after dark and the rate roughly triples. Leave water out of the setup, since they cannot appear in it even though they swim happily once they exist.

A golden version shows up now and then and is worth real coin, and there is a mystic variant that appears far more often than that. Both are the actual reason to build a frog farm, so keep a bug net on the hotbar rather than swinging at everything that moves down there. Statue spawns are a dead end: the net refuses them and they vanish in smoke when you try.

They cook into food, and a stocked kitchen is worth more over a run than the coins are. If you are catching them anyway, keep the surplus instead of selling the lot.