What do you think the "'terrible fish" in the last line of Sylvia Plath's "Mirror" symbolizes?
Symbolism is where an object is used to mean something other than its typical meaning. Given that people can interpret things differently, the defining of a symbol can differ for each individual reader. That said, some symbols have become to be identified as universal (when the critical analysis of the symbol is accepted by all/most readers).
In regards to the "terrible fish" seen in Sylvia Plath's poem 'The Mirror," the fish could represent the aging woman who is struggling to hold onto her past.
The poem speaks of a mirror who shows the reflection of the wall opposite its mounting. The second part of the poem goes onto deepen the meaning behind a reflection seen in both the mirror and the lake. The mirror, now calling itself a lake (personification), recalls the life of the young girl "drowning" in its surface. Instead of the young girl, lost to the depths of the lake, an old woman is seen in the reflection of the lake "rising like a terrible fish."
This fish could symbolize the old woman grasping for her lost youth in the same way a fish opens its mouth at the surface of the water. Both, trapped by the surface of the water, the fish and the old woman are both searching for something outside of the lake.
The mirror does narrate this poem, and so it is personified in this way. However, when the mirror compares itself to a lake, saying, "I am a lake," this is example of metaphor. The mirror explains, in the final lines, "In me she has drowned a young girl, and in me an old woman / Rises toward her day after day, like a terrible fish." In other words, "she" -- the woman -- has watched her youth slowly disappear over the years of looking into this same mirror, and the mirror interprets that as a kind of "drowning" of the woman's youth.
Just as the woman's youth seems to fade away into the mirror, so does old age seem to be getting clearer and closer, rising toward her within the mirror's surface. The "terrible fish," then, is the specter of old age, the total loss of youth and vitality. This is likely why the woman looking into the mirror "rewards [it] with tears and an agitation of hands"; the woman watches herself age slowly, like a huge fish rising to the surface of the water, and she cries and wrings her hands. It is painful to grow old.