於菟
於 is a prefix to indicate respect for the dangerous animal; compare 老 (lǎo, “old; venerable”) in 老虎 (lǎohǔ, “tiger”).
菟 is from Proto-Sino-Tibetan *k-la, from Proto-Mon-Khmer *klaʔ. Cognate with 虎 (hǔ).