“Merely” in Chinese is 只是 (zhǐshì).
这只是马。
This is only a horse.
Example sentence from Tatoeba (CC BY 2.0).
Not to be confused with: 致使 (zhìshǐ, to cause) · 直视 (zhíshì, to look straight at) · 知识 (zhīshi, knowledge) · 芝士 (zhīshì, (loanword) cheese) · 指示 (zhǐshì, to point out)