No copyright renewals are seen for Sam and Friends, if it was published:

https://publicrecords.copyright.gov/search?page_number=1&query=%22sam%20and%20friends%22&field_type=%22title%22&records_per_page=10&sort_order=%22asc%22&registration_class=%22RE%22&model=%22%22

So it's possible the original 1955 Kermit character is Public domain if Sam and Friends and if it was "published" first, and the 1956 registration possibly could be made ineffective since it was not within 3 months or is a derivate work. However this is a bit disputed as Disney still owns later iterations as well as trademarks and this is disagreed upon by some people. So be careful. Feel free to delete though.

This work is in the public domain because it was published in the United States between 1931 and 1963, and although there may or may not have been a copyright notice, the copyright was not renewed. For further explanation, see Commons:Hirtle chart and the copyright renewal logs. Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties. العربية ∙ Deutsch ∙ English ∙ español ∙ français ∙ galego ∙ italiano ∙ 日本語 ∙ 한국어 ∙ македонски ∙ português ∙ português do Brasil ∙ русский ∙ sicilianu ∙ slovenščina ∙ українська ∙ 简体中文 ∙ 繁體中文 ∙ +/−