There are plenty of them out there : almond X peach cross plum X peach ( nectarine is a variety of peach not a cross) , sand cherry X plum , apricot X peach , apricot X plum . in addition pears, quince , Asain pears and quince , apples , flowering crab apples , mountain ash can all crossbreed . squash x pumpkin x gourds all work .
All members of the cabbage family cross .

Do some googling using 'interspecific cross' and 'intergeneric cross' (or something similar) to obtain some information. This sort of natural and human-made manipulation is well documented in agriculture, floriculture, agronomy, horticulture, and many other fields. One of the more famous ones in ornamental use is, of course, X Fatshedera lizei, a cross between Fatsia japonica and Hedera Helix.

After all, if it happens in the animal kingdom, why not plants?

Here is a link that might be useful: {{gwi:996074}}

does anybody know about this happening in the family asclepiadaceae?