Birds from the family Columbidae (e.g., doves and pigeons) produce a fatty, milk-like substance in their crop. The secretion is often referred to as "crop milk." They feed crop milk to their young through regurgitation. Does anyone know of another bird or reptile group (or even mammals) that has evolved such a mechanism; not regurgitation but secretion of a milk-like substance for feeding young? Also, both male and female doves produce crop milk; are there cases where males of a mammal species produce milk?
In mammals there are only two species (known, there may well be others) where males lactate. They are both species of bats and this paper discusses the evolutionary mechanisms that could underlie male lactation. I have ignored human cases because they are more of an unusual occurrence (often brought on by severe dietary stress) rather than an evolved mechanism.
Other non-mammals include Flamingo and Penguins. There is also a species of fish, called a discus fish, which produces a mucus from its skin which the offspring feed on (here). I would hypothesise that this evolved originally for different reasons (defence, scent communication, excretion...) but offspring "discovered" this was slightly nutritious so fed off it, those genetically predisposed to doing such things then had higher fitness and the trait spreads. Evolution of greater or more nutritious mucus production would be achieved because parents that produce more/better mucus would have more offspring that reach maturity.
The paper by Kunz & Hosken is a good discussion on current evidence and the potential for evolution of male lactation.
I hope that answers your question, happy reading!