Ekaterina

Ekaterina (エカテリーナ) is Kongou Mitsuko's pet python.

Poltergeist Arc
She is taken along by Kongou when she was invited to model swimsuits and shows her to the rest of the group while showing herself off in her swimsuit. Most of the girls were freaked out, but Uiharu Kazari took an immediate liking to the snake. Later, Kongou mentions her while she was under MAR's custody and comments that she'll get worried if Kongou doesn't come back soon.

Toaru Kagaku no Railgun OVA 01
She is once again mentioned by Uiharu while she, Saten, and Kuroko were investigating the "Someone's Watching" urban myth.

Intermission
She is mentioned by Kongou when she buys food for her at a pet shop after arriving in Academy City. She also appears with Kongou in the opening panel of Chapter 40 of the Railgun manga.