Foxtails are dangerous for dogs as many of us know, and incredibly "invasive" in California. Check your dogs thoroughly throughout the year, especially in the summer and early fall, and remove any foxtails take special care to look deep into the pads of paws and any furnishings your dog may have.
Foxtails come in several species but all have a spikelet cluster of grass and I have found come in many native seed mixes so look for the following:
Hordeum jubatum Foxtail Barley
Alopecurus Foxtail grasses
Bromus madritensis Foxtail Brome
Setaria Foxtail millets