Think of it like this. If a country border has a straight edge, then all the time will the country sharing said border would also have a straight edge (Ex. Canada-U.S.)
With that being said, Mexico, Guatemala, South Africa, Botswana, Senegal, Oman, Yemen, (arguably) Qatar, Libya, Niger, Sudan, South Sudan, Somalia, Kenya, Tanzania, Uganda, Both Congos, Angola, Zambia, etc. There are a ton of countries missing and I unfortunately don't have the time to name them all.
With that being said, Mexico, Guatemala, South Africa, Botswana, Senegal, Oman, Yemen, (arguably) Qatar, Libya, Niger, Sudan, South Sudan, Somalia, Kenya, Tanzania, Uganda, Both Congos, Angola, Zambia, etc. There are a ton of countries missing and I unfortunately don't have the time to name them all.