horse
騠 centers on a horse, specifically a type of horse. The given meaning is simply 'horse', so the core idea is the animal itself.
騠 combines 馬 (horse) with 是, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where 馬 indicates the meaning category and 是 hints at the pronunciation.
The left side is 馬 (horse), and the right side is 是. Picture a horse that is always correct (是 can mean 'correct') — a reliable, steadfast horse.
For テイ, imagine a horse taking a tea break: tea -> テイ. The horse sips tea while standing correctly (是) beside the stable.