Magic Knight Rayearth (1994)

Season 1: Episode 8

Hikaru takes an estrange monster for caring. She names it Hikaru after her dog. We then find out that ""Hikari"" is actually a ""pet""/monster that Ascot was training. Hikaru spends too much time with Hikari and Fuu and Umi thinks she'd been hurt by a monster or something worse. They go out looking for her, and find a monster by Hikaru. They are ready to kill it, but Hikaru tells them it's Hikari. They don't fight it, but Hikari (or ""Bigot"" the real name) runs amuck on the forest, setting fires, etc. Hikari also knocks Fuu unconcious, and takes a hold Umi. Hikaru is mad, and kills Hikari. They bury Hikari, and experience an earthquake. They hurry off to find their Machines.