Rabbit Substitute?

You could try something like chicken thighs. Rabbit is very mild in flavor but the texture is similar to thigh meat on a chicken or duck. You could try doing it with a young chicken (i.e. cornish hen, poussin) but I would take the skin off of whatever you decide to use.

In Spanish there is a saying: dar gato por liebre.

It literally means to give a cat as a hare, as supposelly they have the same size and similar shape, and would be indistinguishable when cooked.

Albeit you probably should get some pieces to a veterinary in order to check the cat had no diseases (as supposely a rabbit in a butcher would have been tested for), and have guts to eat a pet.

A note on the saying

(Please, feel free to edit this answer and delete this note if you think it's off topic: this is an open collaborative site)

The saying means to trick someone, to deceive: it doesn't have a literal meaning. Culturally, in Spanish speaking countries eating a pet is disgusting. But the saying gives us (Spanish speakers) the idea that both meats would be indistinguishable and, so, be a propper introduction for the answer.

I would agree with the first answer, chicken thighs would probably be the easiest closest match you'd find for rabbit. However, rabbit has a stronger more gamey flavour than chicken so you might want to consider ways of adding that gamey flavour back into the dish.

I'd think about what gamey flavours you could add, perhaps small chunks of venison or even possibly one of the game birds, pheasant, partridge, grouse etc would give you that gamey flavour.