Many Spanish words taken into English have a "J" sounding like "H", but San Jacinto follows a different rule:
- San Jose
- La Jolla
- San Juan
- JiminezJimenez
Why is San Jacinto not pronounced San Hacinto in English?
The languagehat article mentioned in comments shows that the J in Jacinto is under attack in Texas, but it doesn't explain why Texans don't pronounce Jacinto like every other Spanish J word.