Ano Toki Tasukete Itadaita Monster Musume desu. is a Manga/Manhwa/Manhua in english language, Ecchi series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

Some 20 years have passed since the protagonist was summoned to another world and retired from the army. He is still single in his 40s and lives as a low-ranking professor, until one day, an elite 'monster girl' transfers into his class.