That's a very good question. My clever friend @monacoger probably knows. (I also know too but the answer is a bit silly)
Is it not pretty obvious??? The animal on the badge is a hart(another name for a stag, male deer). Watford is based in Hertfordshire (I assume it used to be hartfordshire?) which means deer crossing or something. It really isn't difficult to work out ffs. I think the badge used to have a hornet on it but they changed it.