I have the Ian Evans Lithgow Pottery book which I think is still regarded as the definitive Lithgow work. There is a majolica jug (not mug) with raised fern pattern listed in the 1889 catalogue and a colour photo is shown of one on page 7. However it is solid majolica glaze, not with the fern picked out in green; the caneware type body and handle on your example are not like anything else to be found in the book. I have never heard of Lithgow making this kind of frog / toad / surprise mug, but I am no expert.