"Plow horse" is a term often used to describe someone who works tirelessly and persistently towards a goal. There are several synonyms that can be used in place of "plow horse" to convey a similar meaning. One such synonym is "workhorse," which implies a person who works steadfastly and often without complaint. "Stalwart" is another synonym that works well, as it highlights someone who is unwavering in their efforts and reliable in achieving their goals. "Persevering" is also a great substitute for "plow horse," as it emphasizes a person who is dedicated and committed to seeing a task through to completion. Ultimately, these synonyms capture the essence of someone who is determined and hardworking in the pursuit of their ambitions.