It's definitely a prop, but probably inspired by an existing item. Try maybe to search for fish ashtrays or ornaments in general, not specific mackerel (since I think they only called a mackerel for the sketch). Pretty sure there's something out there which doesn't look 1:1 like the one in the sketch, but at least similar enough to it.

Alternatively, how about to try to recreate it, like out of clay for example? It's maybe a bit difficult to get all the details right but it could be a fun challenge. Maybe you will also find an fish ornament and a fitting ashtray which can be modded together to make it look like the one from the sketch. (Now that I think about it I would actually try this out myself.)