Answer:
The figure of speech is ALLUSION.
The intended effect is humour
Explanation:
Based on the given figurative language in the line, "Desperate to turn the farm's fortunes around, Father bought a new plough horse and named him Hercules.", the speaker is making an allusion to the strong warrior Hercules by naming his plough horse after him.
This is supposed to make the horse work harder and make the farmer more successful, but its effect is humour.
