i buy the present in the shop and it come out the imp, i dont know its a bug or its on purpose, but i think the imps were only spawned by the fiend baby

It's on purpose and it is really rare because it is a buffed puffshroom from pvz

The Imp can show up guaranteed if you buy Mystery Gift after exhausting the familiar pool.

Which is probably what happened in this case considering your run.

But yeah it's an intended feature.

thank you